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LABORATORY EXERCISES 



rO ACQ MPANY 



CAMIAKT AND CIIUTK'S 

FIRST PKINCIPLLS Ol PHYSICS 



i:V 

ROBERT vv. FULLER 

AND 

RAYMOND B, BROWNLEE 

II KVBSANT uk. ii SCHOOL, NEW vokk CITY 



"*.«k«>« 



Boston 
Al.l.YN and BACON 

i 9 i ! 



< 



COPYRIGHT, 1912 and 1913, BY 

ROBERT W. FULLER 
AND RAYMOND B. BROWNLEE 



©CI.A343366 



PREFACE 

In the preparation of a Physics laboratory manual, it is nec- 
essary to take into account the diversity of courses and equip- 
ment in different schools. The individuality of the teacher 
and the limitations of equipment have been recognized in the 
selection and treatment of topics, in these Exercises, and a wide 
choice of experiments has been provided, only a few of which 
require highly specialized apparatus. Where such apparatus 
is decidedly superior to that commonly used, specific directions 
for its preparation have been given in footnotes. Experiments 
which have proved their value in generally accepted courses 
have been retained with such simplifications as seem desir- 
able to secure directness. Other experiments which repre- 
sent the more modern trend in Physics teaching are introduced 
in considerable number to enrich the course. 

Many students lose the value of the laboratory period be- 
cause it is spent in following directions in a purely mechanical 
way, while they wonder why the particular problem on which 
they are engaged should be done at all. In order to settle 
this question in the student's mind, the authors have been in 
the habit of furnishing their students with .an introductory 
paragraph for each experiment. This introduction connects 
the experiment with the pupiFs experience and furnishes defi- 
nitions of such terms as are necessary to the understanding of 
the directions. This introductory paragraph also permits the 
laboratory experiment on a particular topic to be given either 
before or after the subject is discussed in class, as the instructor 
may desire. 

Following the plan of the best laboratory manuals, definite 
provision has been made for recording observations and calcu- 
lated results either in tabulations or in simple diagrams. This 
plan permits the student's record to be made in a minimum 
time and leaves a larger proportion of the period available for 
actual laboratory work and the consideration of results than is 
possible w r hen a running record is made. It also reduces the 



iv PREFACE 

time necessary for the instructor to judge the accuracy of the 
student's results, as well as his grasp. of the principles involved. 

The Conclusion always calls for a definite answer to the 
question which- is raised in the Object of the experiment. 
Other important facts or principles which may be deduced in 
connection with the experimental work are subjects for ques- 
tions in the Discussion. These may be answered orally as the 
instructor passes from student to student, or, in large classes 
particularly, the answers may be written in the note-book. In 
any case, these questions direct attention to the salient points 
of the experiment and should be taken up in the quiz on the 
laboratory work. 

The authors wish to express their grateful appreciation of 
the assistance which they have received in the preparation of 
this book. Among their associates in the Stuyvesant High 
School they are particularly indebted to the principal, Dr. E. 
R. von Nardroff for valuable suggestions and for the stimulus 
which his interest in the experiments has afforded; to their 
colleagues in the Physical Science Department, particularly 
Mr. J. G. Baier and Mr. H. W. Mott, for many helpful criti- 
cisms and suggestions; and to Dr. H. E. Fritz, for valuable 
assistance in the preparation of the drawings. Many of the 
drawings have been made by the following students : Charles 
E. O'Rourke and Harold Jay, of the Stuyvesant High School, 
and John G. Smith, of the Geneseo Normal School. In connec- 
tion with particular experiments, acknowledgment is made to 
the teachers who rendered assistance in these experiments. 
Thanks are tendered also to Professors Carhart and Chute for 
permission to use several cuts (Figs. 3, 35, 68, and 112) from 
the " First Principles of Physics " ; to Professor W. H. Timbie 
for the use of the Resistance Table (p. 315) from his "Ele- 
ments of Electricity"; and to the L. E. Knott Apparatus 
Company for the use of several cuts. 

The authors will gladly receive criticisms and suggestions 
from teachers who may use the Exercises in their classes. 

R. W. F. 
R. B. B. 

February, 1913. 



CONTENTS 



Sug 


^gestions to the Instructor ..... 


1 


Directions to Students ...... 


9 




Mechanics 




EXPE 


RIMENT 




1. 


Metric Units of Measurement .... 


18 


2. 


Properties of Materials 


. 21 


3. 


Measurement of Bodies ..... 


24 


4. 


Volume Measurement of an Irregular Body 


30 


5. 


Density ........ 


. 32 


6. 


Elasticity — Hooke's Law . . 


34 


7. 


Tenacity of Wire . . . . 


37 


8. 


Relation between -Pressure and Depth 


40 


9. 


Archimedes' Principle ..... 


43 


10. 


Law of Flotation ...... 


45 


10. 


(Alternative) Law of Flotation 


46 


11. 


Specific Gravity of Solids . 


48 


12. 


Specific Gravity of a Liquid (Bottle Method) 


50 


13. 


Specific Gravity of a Liquid (Hydrometer Method) 


52 


14. 


Specific Gravity of a Liquid (Hare's Method) 


55 


14. 


(Alternative) Specific Gravity of Liquids (Balancing 






Golumns) ........ 


• 58 


15. 


Density of Air ....... 


62 


15. 


(Alternative) Density of Air . 


65 


16. 


Boyle's Law ........ 


68 


17. 


Measurement of Gas Pressure . 


71 



VI 



CONTENTS 



EXPERIMENT 



18. Water Pumps 

19. Principle of Moments 

20. Lever Arm of a Force 

21. Composition of Several Parallel Forces 

22. Four Forces at Right Angles 

23. Parallelogram of Forces . 

24. Resolution of Forces 

25. Force at the Center of Gravity of a Body 

26. Pendulum 

27. Inclined Plane 

28. Pulleys .... 

29. Wheel and Axle 

30. Mechanical Efficiency of Machines 

31. Coefficient of Friction 



PAGE 

74 

77 

80 

82 

85 

87 

90 

93 

96 

99 

102 

107 

110 

113 



Sound 



32. Vibrations of a Tuning Fork 

33. Velocity of Sound in Air . 

34. Sympathetic Vibrations 

35. Wave Length of a Sound . 

36. Laws of Vibrating Strings 



116 
120 
122 
125 
129 



Light 

Power - 



37. Measurement of Candle 

Photometer ..... 

37. (Alternative) Measurement of Candle Power 

ford Photometer ..... 

38. Law of Reflection of Light 

39. Images in a Plane Mirror 



Jolly or Bunsen 



• Rum- 



133 

136 
139 
142 



CONTENTS 



Vll 



EXPERIMENT 

40. Reflection in a Concave Mirror 

41. Reflection in a Convex Mirror . 

42. Refraction through a Glass Plate 

43. Refraction through a Prism 

44. Index of Refraction . 

45. Total Reflection 
46 A. Study of a Converging Lens . 
46B. Focal Length of a Converging Lens 

47. Conjugate Foci of a Converging Lens 

48. Magnifying Power of a Lens 
49A. Astronomical Telescope 
49B. Compound Microscope . 

50. Dispersion of Light by a Prism 



144 
148 
149 
151 
153 
155 
159 
164 
166 
169 
172 
176 
179 



Heat 

51. Fixed Points of a Thermometer . . . .181 

52. Phenomena of Boiling . . . . . .185 

53. Coefficient of Linear Expansion . . . .190 

54. Coefficient of Cubical Expansion . . . .193 

55. Increase in Volume of a Gas at Constant Pressure . 197 

56. Increase in Pressure of a Gas at Constant Volume . 201 

57. Law of Heat Exchange ...... 205 

58. Specific Heat of a Metal 209 

59. Cooling through Change of State . . . .213 

60. Melting Points and Boiling Points . . . .216 

61. Heat Changes during Solution and Evaporation . . 220 

62. Heat of Fusion of Ice . . . . 223 

63. Heat of Vaporization ...... 226 

64. Dew Point 230 



Vlll 



CONTENTS 



Magnetism and Electricity 

EXPERIMENT PAGE 

65. Magnetic Induction 232 

66. Magnetic Lines of Force ...... 235 

67. Development of an Electrostatic Series . . . 238 

68. Simple Cell 241 

69. Two-fluid Cell 244 

70. Electroplating " 247 

71. Electrotyping .250 

72. Storage Cell 252 

73. Laws of Resistance ...... 255 

74. Effect of Temperature on Resistance . . . 258 

75. Internal Resistance of a Cell 261 

76. Grouping of Cells ....... 263 

77. Resistance and Current in a Divided Circuit . . 266 

78. Resistance by Substitution . . . . . 269 
Heating Effect of an Electric Current . . . 272 
Study of an Incandescent Lamp .... 276 
Lines of Force around a Conductor .... 278 
Electromagnet . . . . . . .281 

83. Electric Bell 284 

84. Telegraph Instruments ' . . . . . . 286 

Operation of an .Electric Motor .... 288 

Power and Efficiency of a Motor .... 290 

Relation between Fall of Potential and Resistance . 296 

Resistance by the Wheatstone Bridge . . . 298 

Induced Currents ....... 302 

Study of a Dynamo ....... 304 



79 
80 
81 
82 



85 
86 
87 
88 
89 
90 



CONTENTS 



IX 



Appendix 

PAGE 

I. Important Numbers and Equivalents . . . 309 

II. Properties of Materials 310 

III. Density of Water 312 

IV. Index of Refraction * 312 

V. Electromotive Force of Cells . . . .312 

VI. Table of Natural Sines and Tangents . . .313 

VII. Size and Resistance of Annealed Copper Wire . 314 

VIII. Specific Resistance and Temperature Coefficient . 315 



INTRODUCTORY 

SUGGESTIONS TO THE INSTRUCTOR 

Selection of Experiments 

Scope of the Experiments. — The experiments in this book 
provide a wide range of laboratory work for an elementary 
course in Physics. The exercises have been selected on the 
basis of their educational value to the student. Their aim is 
to impart to him certain fundamental principles, to acquaint 
him with some physical phenomena qualitative in character, 
and to show the operation and the use of practical devices or 
instruments that are applications of physical principles. 

The authors have not hesitated to omit from their list 
certain well-known experiments which have persisted in many 
elementary courses, rather by inertia than because of any 
special interest or value to the beginner. On the other hand, 
it is impossible to include in a small book all the experiments 
of merit suitable to a first course in Physics. Yet, from 
those given, it will be possible for any instructor to make a 
selection of the experiments which the great majority of 
Physics teachers include in their courses, so as to afford a 
well-balanced laboratory training, both interesting and instruc- 
tive to the student. 

Recommended Lists. — Only the institutions most favored as 
to laboratory time will be able to complete in one scholastic 
year all the experiments outlined in this book. Any choice 
of experiments must depend upon the apparatus available and 
upon the laboratory conditions. To fit the usual laboratory 
equipment and to meet the time limitations of most first 
courses in the subject, the authors suggest the following list 
of thirty-five experiments as affording a good training in those 

1 



2 GENERAL SUGGESTIONS 

fundamentals of the science most suitable for laboratory in 
struction : 

Fundamental Course 

Mechanics: Exercises 3, 4, 5, 8, 9, 10, 11, 19, 23, 25, 26, 27. 
Sound: Exercise 35. 

Light : Exercises 37, 38, 39, 42, 43, 46 A, or 46 B and 47. 
Heat: Exercises 51, 58, 59, 61, 62. 

Magnetism and Electricity : Exercises 66, 68, 69, 70, 80, 81, 82, 
83, 84, 85, 89. 

The following ten exercises will supplement the above, 
particularly for those students whose ability enables them 
to do a maximum amount of work : 

Mechanics, 6, 13 or 14, 28, 29 ; Heat, 57 ; 
Sound, 34 ; Magnetism and Electricity, 72, 

Light, 44; 78 (or other experiment 

on resistance), 90. 

The following sixty exercises are suggested as a more 
extended course for those institutions favored with about 
double the laboratory time usually allotted to the first course : 

Extended Course 

Mechanics : Exercises 1, 3, 4, 5, 8, 9, 10, 11, 15, 16, 17, 19, 
20, 23, 24, 25, 26, 27, 28, 29, 30. 

Sound : Exercises 32, 34, 35. 

Light : Exercises 37, 38, 39,42, 43,44, 46 A, or 46 B and 47, 48. 

Heat: Exercises 51, 52, 57, 58, 59, 60, 61, 62, 63. 

Magnetism and Electricity : Exercises 65, 66, 68, 69, 70, 72, 
73, 78, 79, 80, 81, 82, 83, 84, 85, 86, 89, 90. 

The authors recommend the following list of experiments 
for girls, especially for those not intending to go beyond the 
high school. Most of these experiments have been selected be- 
cause of their close relationship to the practical affairs of life. 

Mechanics: Exercises 1, 2, 3, 6, 8, 9, 12 (or 13 or 14), 17, 18, 
23, 26, 27, 28. 



TO THE INSTRUCTOR 3 

Sound: Exercises 34 (or 35), 36. 
Light: Exercises 37, 38, 39, 49, 50. 
Heat: Exercises 51, 52, 59, 60, 61, 64. 

Magnetism and Electricity : Exercises 65, 68, 70, 79, 80, 82, 
83, 84. 

A number of interesting and valuable experiments do not 
appear in any of the preceding lists, but it is hoped that some 
of them will be taken from time to time either as substituted 
or as additional exercises. A limited amount of variation 
from year to year adds interest and vitality to any laboratory 
course. Many of the experiments just referred to will meet 
the needs of those instructors who desire to give more time to 
certain divisions of the subject. 

Order of Experiments. — The order in which the divisions of 
the subject are taken should depend upon the aim of the 
course and the conditions under which it is given. In their 
own work the authors find the most satisfactory order to be 
Mechanics, Heat, Sound, Electricity, and Light. In most 
syllabi, however, the subject of Light precedes that of Electricity. 
In the view of many, the experiments on Heat are best adapted 
to the student's powers after he has finished the experiments 
in Mechanics. 

Time required for Experiments. — A majority of the experi- 
ments are designed to take from 80 to 90 minutes of laboratory 
time, including the writing of the note-book record. Some of 
the shorter ones will require but half of that time, or a single 
school period. Even if a double laboratory period is not 
available for the longer experiments, the directions have been 
written so that the experiments can be done successfully in 
two single periods. The system recommended for the note- 
book record saves time in securing the observational data. 
Especial care has been taken not to overload the student with 
more manipulations and observations than would be reason- 
able for an average rate of work within the time allotment. 



4 GENERAL SUGGESTIONS 

The Experimental Directions 

Aim. — At first sight it may seem that the directions for the 
experiments have been written in a rigid form which may 
hamper the individuality of the teacher using them. With 
the possible exception of the placing of the tables of observa- 
tions and calculated results, it will be found that the directions 
and their requirements are in accord with the usages which 
have become generally established as leading to intelligent and 
efficient laboratory work. 

The five main divisions of the printed directions are 
"Introductory," "Experimental," "Calculated Eesults," "Dis- 
cussion," and "Conclusion." Certain suggestions as to these 
divisions appear in the paragraphs that follow. 

Introductory. — The paragraphs under this heading in the 
printed directions serve several purposes. First, they awaken 
the student's interest in the problem to be studied by reference 
to applications of Physics more or less familiar to him. Sec- 
ondly, the introductory statements show the relation between the 
practical applications and the laboratory problem to be solved. 
In some cases the paragraphs furnish a little theoretical in- 
formation, necessary for the intelligent performance of the 
experiment. All that is required of the student is that he read 
and understand this introductory matter — usually a task of a 
few minutes. It is not expected nor is it desired that the in- 
troductory matter be copied into the note-book. 

The authors offer no apology for the paragraphs introduc- 
tory to the experiments. They have simply put in written 
form those preliminary remarks that many instructors find 
desirable to make when the class assembles for the experiment. 
It is felt that the written form has the advantage of being al- 
ways available for the student's reference. 

Experimental. — Whenever the length and character of the 
experiment permits, the laboratory problem is presented as a 
whole to the student. With the general plan in mind, he is 
able to do the experiment with greater self-reliance and effi- 






TO THE INSTRUCTOR 5 

ciency than can be obtained from the slavish following of 
detailed directions with little grasp of their intent. 

In some experiments, however, detailed directions must be 
given to secure the successful imparting of a series of experi- 
mental facts. In such cases the divisions are made as few as 
possible and their meaning made clear by brief directions, a 
little supplementary information, and questions that the aver- 
age student should be able to answer from his experimental 
observations. 

The students are directed to place the data gathered in the 
experiment in a table of observations near the top of the left- 
hand page of the note-book record. The form for this table is 
usually furnished, and it is strongly recommended that the 
student write the form in the note-book before making any of 
the measurements. This procedure provides for the orderly 
recording of the data as soon as it is obtained, and insures the 
completion of the experimentation within the laboratory hour. 
There is economy also of the instructor's time, as he can 
quickly note the rate of progress of the individual and check 
inaccuracies in the readings. 

^Yith most experiments only one set of readings is indicated 
in the tables of observations, but the instructor desiring more 
can increase the number of columns at the right. In the 
opinion of the authors, much time is wasted by requiring the 
duplication of readings by the elementary student of Physics, 
unless in work where personal errors are large. 

Drawings. — After the observations are completed, the 
student is directed to make sectional or outline drawings from 
his apparatus so as to show that he understands its arrange- 
ment and operation. Many of the illustrations in this book 
have been made from drawings made by students in the regular 
course of their laboratory work. Such drawings will indicate 
to the users of this book the methods of representing labora- 
tory apparatus by simple outline drawings. The development 
of a simple scheme of sectional representation is within the 
power of any student and will prove most useful to him. 



6 GENERAL SUGGESTIONS 

Descriptions. — The table of observations and the sectional 
drawings render unnecessary long and elaborate descriptions of 
the experimental work. All that is asked is a brief but clear 
statement of the general method of the experiment and the 
recording of any experimental facts not shown by the draw- 
ings nor provided for in the table of observations. In the 
last few years it has become more and more recognized that 
the chief function of the laboratory note-book is to show the 
essentials of an experiment and not to provide useless drudgery 
for the student. 

Calculated Results. — Preceding the table of calculated results 
occurring in many experiments, are found directions for mak- 
ing the calculations. The authors have not hesitated to fur- 
nish information to aid the student in making the calculations 
when these are rendered more intelligible thereby. 

The directions call for the placing of the table of calculated 
results at the top of the right-hand page of the note-book record. 
The calculations themselves should be made directly # below the 
table. These requirements secure prominent and convenient 
locations for the making of the computations and the orderly 
recording of the results. The student can tell from the tabular 
form what is expected of him in the way of calculations and 
know? when his work is finished. The instructor is enabled 
to check quickly the recorded results and to point out during 
the laboratory period sources of error. 

Discussions. — Under this division the student is directed to 
answer any italicized questions occurring in the experimental 
directions or the questions under the printed heading, Discus- 
sion. Thus the theoretical considerations of the experiment 
are brought together ready for reference or correction. 

Conclusions. — The student is either required to state for 
himself the formal conclusion justified by the experimental 
facts, or to complete a partial statement by filling in the in- 
dicated blanks. The latter method is preferred in those cases 
where a complete and well-worded conclusion is difficult for 



TO THE INSTRUCTOR 7 

the student to formulate. The vital part of the statement 
must be furnished by the student and requires thought on his 
part. 

Method of Laboratory Work. — Many of the advantages of 
having the note-book record follow a definite plan have been 
discussed under the topics preceding this. Tabular forms for 
the observations and the calculated results are appreciated by 
many instructors as leading to that economy of laboratory 
time which gives the best opportunity for experimentation and 
reflection. The forms for such tabulations may be written in 
the note-book prior to the laboratory hour and the general plan 
of the experiment studied. 

The authors believe that it is not only permissible, but highly 
desirable, for the student to know before he comes into the 
laboratory what he is to do. They require their own students 
to carefully study the experiment and to write the blank table 
of observations in the note-book before coming to the laboratory. 
Except in the case of very complicated experiments, the student 
is not allowed to have the experimental directions before him 
until he has taken all readings and completed his drawing and 
description. He is then allowed to refer to his direction sheet 
for guidance as to his calculations and conclusions. It has 
been found that under this plan the work in the laboratory is 
more intelligent and less of the " cook-book " order. Further- 
more, schools having only single laboratory periods may be 
certain of having the readings taken and the experiment 
described during the laboratory period, while calculated results 
and conclusions may be worked out the next day either in 
laboratory or classroom, or, if desired, done as part of the home 
lesson for the day following that of the laboratory period. 

No factor contributes more to the success of a laboratory 
course than having the apparatus tested and entirely ready for 
the student when he enters the laboratory. Then only is it 
possible for him to put the apparatus together and start its 
operation without loss of time, so that the readings can be 
made comfortably within the period. 



£ GENERAL SUGGESTIONS 

}iiNafce*book Directions. — On page 16 there will be found 
brief instructions intended for the student and relating to the 
form of the note-book record. Any orderly plan must have 
definiteness ; so it becomes necessary to designate left-hand 
and right-hand pages for certain purposes. These directions 
may reverse the usage of some instructors, but it is hoped 
that they will realize it makes little difference whether the 
left-hand page or the right-hand page serves a certain purpose, 
so long as there is a definite systematic plan to make the note- 
book record a help to the student, and to make the ever present 
and laborious task of note-boot correction easier for the 
instructor. 

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DIRECTIONS TO STUDENTS 



Balances 

Construction of Platform Balances. — The platform balance ot 
trip scale is a simple, equal arm lever in which the vertical 
displacement of either arm is indicated by a pointer swinging 
across a horizontal scale. When the pointer swings approxi- 
mately equal distances on each side of the? center division on 
the horizontal scale, the two lever arms are balanced and the 
scale is said to be in equilibrium, 




Fig. I. Platform Balance. 

The construction of the trip scale is shown in Figs. 1 and 2 
on this and the following page. This convenient instrument 
for weighing is too often misused in the physical laboratory 
and poor results obtained with it. With the observance, how- 
ever, of a few simple precautions, rapid, accurate weighings 
can be made with this piece of apparatus. 

Adjustment of Platform Balances. — Before weighing always 
see that both platforms are clean Then touch lightly one 

9 



10 



GENERAL SUGGESTIONS 




Fig. 2. Sectional View of Balance. 



platform and note whether or not the pointer swings freely 
and equally on each side of the center line of the scale. The 
pointer should oscillate at least two divisions to the right and 
to the left. In too short swings the friction in the bearings 

makes the scale rela- 
tively less sensitive. 
Therefore the point- 
er's coming to rest at 
the center point is no 
sure indication that 
the two arms of the 
scale are balanced or 
in equilibrium. 

In case the pointer 
swings to a distinctly 
greater distance on 
one side of center, 
turn the thumb nut which is just below the center, so that the 
nut moves a little distance towards the side of the lesser swing. 
Again note the swings. When they are approximately equal 
on both sides of center, the scale is adjusted for weighing. 

Handling of Weights. — Place the object to be weighed on 
the left-hand platform .or pan and the weights on the right-hand 
platform. In adding or removing weights, prevent with the 
left hand the movement of the pans until the change of 
weights has been made. In this way avoid jarring the balance 
and injuring the knife-edges. 

For the first weight select the one which in your opinion is 
about equal to the object being weighed. If this weight is too 
small, take it off and replace it with the next larger one. 
Continue in this way until you have the largest weight which 
is lighter than the object. Then add the next smaller weight. 
Time-saving weighing means the systematic use of the next 
smaller or the next larger weight, as the case may be, until 
the scale is balanced. 

In practice the graduated beam with its rider enables one to 



TO STUDENTS 11 

dispense with the smaller weights. If the beam is graduated 
for 5 grams, the 1-gram and the 2-gram weights are not used ; 
with a 10-gram beam, the weights below 10 grams are not 
necessary. By means of the graduated beam, these smaller 
weights are found by moving the rider to the right until the 
balance is in equilibrium. Note carefully on which side of 
the rider the reading should be made, and remember that 
the reading can be made to tenths of a gram. 

When the correct weight is obtained, count carefully the 
weights on the right-hand pan and add the weight indicated 
on the beam. Record this total weight at once in the labora- 
tory note-book. 

Return the weights to their block, or case, counting as you 
do so. Add the weight indicated on the beam and check the 
weight recorded in the note-book. Remove the object from its 
scale pan. A scale left with the arms unequally balanced soon 
loses its sensitiveness, owing to unnecessary wear on the 
bearings. 

Beam Balances. — Another form of balance much used in the 
physical laboratory is the beam balance. The beam in this 
case rests at its center point on a knife-edge, or a wedge, sup- 
ported on a vertical stand. Pans are suspended on the ends 
of the beam either by hooks, or in the more expensive kinds by 
stirrups which rest on knife-edges. A vertical pointer indi- 
cates on a small graduated scale the oscillations of the beam. 
Some beam balances have on one arm of the beam a rider, 
which slides along a graduated scale and thus indicates the 
smaller weights. To avoid dulling the knife-edges, there is 
often a device which lifts the beam off the knife-edges when 
the balance is not in use. The pan arrest similarly lifts the 
bow and stirrup suspension from off the knife-edges on the 
ends of the beam. 

The specific gravity balance is usually a beam balance which 
has a shorter suspension for one of the pans. From a hook on 
the under side of this pan are suspended objects which are to 
be weighed in a liquid. 



12 GENERAL SUGGESTIONS 

The hornpan balance is simply a beam balance, which is sup- 
ported yertically from a hook hung on a ring stand or held by 
the hand. 01 woJfed ■ bl 

Spring Balances. — A spring balance measures the mass of a 
body by the elongation of a spiral spring. The weight is in- 
dicated on a graduated scale by a pointer attached to a draw- 



on the free end of the spring. Attached to the drawbar 
is a hook on which is suspended the object to be weighed. 

The spring balance is made to read correctly in vertical 
position, with the hook downward. The weight of the draw- 
bar and hook should be sufficient to bring the pointer to 
the zero mark on the graduated scale. If the pointer does 
not stand at zero with no load on the balance, a correction 
must be made to the weight registered on the scale in order to 
get the true weight of the object. The inconvenience of mak- 
ing these corrections may sometimes be avoided by wrapping 
about the shank of the hook a strip of sheet lead, sufficient in 
weight to bring the pointer to the zero point of the scale. 

The friction in a spring balance tends to make less accurate 
the readings in the first portion of the graduated scale. At the 
other end of the scale, when the spring is near its maximum 
stretch, the elongations are not quite proportional to the 
heavier weights added. Accordingly the most accurate read- 
ings with a spring balance are those obtained in about the 
middle portion of the graduated scale. 

In some experiments the spring balance is used to measure 
the pull or force exerted upon its spring. When used for this 
purpose it is termed & dynamometer. 

Sensitiveness of a Balance. — The sensitiveness of a balance 
may be defined as the smallest difference which is indicated 
by the balance with a given load. The trip scale should be 
sensitive to at least the tenth of a gram with an ordinary load, 
%.e. show a difference between 50.6 and 50.7 grams. A good 
hornpan balance indicates weights within the hundredth of a 
gram (1 centigram) while an accurate chemical balance is sen- 
sitive to a ten-thousandth of a gram (tenth of a milligram). 



TO STUDENTS 30 13 

Relative Advantages of Platform and Beam Balances. — The 

platform balance, while it is easy to keep clean and can stand 
much usage, is usually not so sensitive as the beam balance. 
The broad platforms, however, are very convenient for weigh- 
ing bulky, unstable objects, and the oscillations of its bestaf 
are easily controlled. 

The sensitiveness of a beam balance is gained at the expense 
of stability and durability, for the beam is easily displaced and 
the knife-edge suspension becomes dulled by use. On this ac- 
count great care should be taken not to jar the balance nor 
allow the beam to oscillate too rapidly. The weights should 
be placed gently upon the pans and removed when the pans 
are at rest {i.e. supported by the pan arrest or by the hand). 

Were it not for the awkwardness and carelessness of some 

students, the beam balance would always be most desirable for 

rapid, accurate weighings in the physical laboratory. 

r.Eqoroo 

£ ffoJTT uTIOO 

Electrical Measuring Instruments row 

ton 

The instruments used for measuring the strength or the 

pressure of an electric current have very delicate parts and 
may be easily ruined by either rough usage or excessive 
current 

Before using any galvanometer or other meter the student 
should assure himself that it has the proper scale range and 
current-carrying capacity for the work in hand. He must 
further so connect his apparatus that the instrument will not 
be upset or pulled out of place by any change in connections 
made during the experiment. As the several instruments that 
the student may be called to use in his experiments differ in 
their sensitiveness, method of connection, and method of read- 
ing, each kind will be briefly discussed by itself. In reading 
all instruments, tenths of the smallest divisions, should be es^ 
mated. 

Tangent Galvanometer. — This consists of a compass needle 
mounted at the center of a hoop, on which is wound the wire 



14 GENERAL SUGGESTIONS 

which is to convey the current. This is the most rugged of 
the instruments, but the pivot is likely to be bent by dropping 
or violently jarring the instrument. Where there are a num- 
ber of binding posts, to permit the use of different numbers of 
turns of wire, find out from the instructor which posts to use 
and the number of turns of wire included between them. In 
order to read the instrument accurately, it should be so placed 
on the table that it will be possible to look directly down on 
the needle. The instrument should be carefully turned until 
the needle is in the plane of the coil. 

D'Arsonval Galvanometer. — The moving part of this instru- 
ment is a light coil of wire, suspended between the poles of a 
permanent magnet by a fine wire or ribbon through which the 
current passes. This suspension is exceedingly thin, so that 
even a slight shock to the instrument will break it and a 
comparatively small current will melt it. The instrument is 
commonly provided with a clamping device which takes the 
weight of the coil off the suspension when the galvanometer is 
not in use. 

In setting up the galvanometer, keep the coil clamped until 
you are ready to connect to the source of current. Then make 
sure that the instrument is leveled in such a way that the coil 
does not rub against any part of the instrument but hangs per- 
fectly free. The method of reading the deflections for the 
particular instrument you are using will be explained by the 
instructor. 

It is exceedingly important that only a very small current 
pass through the coil of the instrument. On this account, the 
galvanometer should have either a coil of high resistance in 
series with it or a low resistance shunt across the terminals for 
most experiments. Such additions to the instrument should 
be made either by the instructor previous to the laboratory 
hour or under his immediate direction by the student. 

Ammeter. — The commercial form of this instrument is 
usually a d'Arsonval galvanometer provided with a shunt of 



TO STUDENTS 15 

such resistance that the deflections of the needle give the num- 
ber of amperes directly. The coil is pivoted instead of being 
suspended, but the instrument must be guarded against falls 
and shocks just as a fine watch would be. 

Before connecting the ammeter in circuit, be sure that its 
range is sufficient for the current to be measured. If the in- 
strument has more than one range, always connect for the largest 
range first, and then change the connections to those for a 
smaller range, if the readings indicate that this can be safely 
done. 

If the ammeter has an external shunt, be sure that the con- 
nections between the shunt and the instrument movement are 
tight. A loose contact will certainly make an incorrect read- 
ing and may burn out the instrument. 

Connect the terminals of the instrument in series with the 
circuit. If connected in shunt with the other apparatus, the 
resistance of the instrument is so small that the movement will 
probably be burned out. 

In every electrical circuit, there should be a switch that can be 
opened instantly if there is the slightest indication of too much 
current for the instruments or any other part of the apparatus. 

Voltmeter. — This is similar to the ammeter in construction, 
but has a high resistance in series with the movement instead 
of a shunt across the movement. The voltmeter measures 
pressure, while the ammeter measures current flow. 

The same precautions for handling and for the selection of a 
proper scale range are to be observed as in the case of the am- 
meter. 

Connect the voltmeter across (in shunt with) the circuit or 
that part of the circuit in which the voltage drop is to be 
measured. 

Resistance Box. — The voltage applied to a resistance box 
should never be great enough to cause more than 0.1 ampere to 
pass through the box. 



16 GENERAL SUGGESTIONS 

The Laboratory Note-book 

Unless other directions are given by the instructor, the fol- 
lowing plan should be followed in recording experiments in 
the note-book. 

Number of Experiment. — Place to the left and at the top of 
the left-hand page. 

Date of Experiment. — Place to the right and at the top of 
the left-hand page. 

Title. — Place immediately below the number and date. 

Object. — Place directly below the title. 

Tables of Observations. — Place immediately below the object. 
In case the instructor desires the duplication of the observa- 
tions, make the necessary number of parallel columns at the 
right. Always record the measurements, as soon as made, in 
the tabular form. Decimals should be used, rather than com- 
mon fractions. 

The number, the date, the title, the object, and the table of 
observations should be written in the note-book before the 
experimental work is begun. 

Drawings. — Place on the left-hand page clear sectional 
drawings showing the arrangement and operation of your ap- 
paratus. In making a sectional drawing, imagine a vertical 
plane passing through the middle of your apparatus ; then 
imagine your paper to be in the position of this plane. Draw 
lines where the paper would touch the intersected apparatus. 

Descriptions. — Place these usually on the left-hand page and 
shorten your work by referring to your drawings. A simple, 
clear account of the general method of the experiment is prefer- 
able to an elaborate description. 

Table of Calculations. — Place at the top of the right-hand 
page before making any of the calculations. Do the mathe- 



TO STUDENTS 17 

matieal work involved, immediately below the table, and record 
the results as soon as obtained in the tabular form. 

Discussion. — Under this heading on the right-hand page, 
answer any italicized questions occurring in the experimental 
directions as well as the questions under the printed heading 
of " Discussion." If more room is necessary, continue on the 
next right-hand page. 

Conclusion. — Place under this heading on the right-hand 
page, immediately following the Discussion. 

Introductory. — It will pay you to read and understand this, 
before beginning the experimental work. It is not to be copied 
into the laboratory note-book. 



LABORATORY EXERCISES 



EXPERIMENT 1 

Metric Units of Measurement 

OBJECT. To become familiar with the units of metric measure- 
ments commonly used in scientific work. 

Apparatus. Meter stick; scissors; small graduate (50 or 
100 c.c.); large graduate (500 or 1000 c.c.); liquid quart meas- 
ure; small wide-mouth bottle; tumbler; platform balance ; metric 
weights ; 1 lb. weight. 

Material. " Oak tag," or some other kind of stiff paper; 
mucilage, or paste. 

Introductory : 

The Metric System is the official system of units of 
measurement in most civilized countries. It is the system 
used in scientific work in the United States. The unit 

100 MILLIMETERS = 10 CENTIMETERS = 1 DECIMETER = 3.937 INCHES. 



1| 2 


3 


4 


5 


6 


7 


8 


£ 


10 






















I 






Mil 


INI MINIM 


Mil 


MM 


1 1 1 1 1 I 1 


1 


MlMMM 


ll II 1 1 1.1 


inn mi 


MIlllMI 


11.11 Mil 


llllllill 




I 


1 1 1 1 1 1 1 


1 1 1 1 


1 1 1 1 


1 1 1 1 


I III 














i 


2 


3 


4 



INCHES AND TENTHS 



Fig. 3. 



of this system is the meter, and standard bars with this 
distance marked on them are preserved for reference by 
various governments. 

The Metric System is a decimal system and therein 
lies its great convenience. The meter is subdivided into 

18 



METRIC UNITS OF MEASUREMENT 19 

ten parts, each of which is termed a decimeter ; the hun- 
dredth of a meter is a centimeter; the thousandth of a 
meter, a millimeter. From these fundamental units, the 
units of surface, volume, and weight are derived. 

The meter measures 39.37 inches. 

Experimental : 

At the top of the left-hand page of the laboratory note- 
book put the number and title of the experiment and the 
date. Then state the object of the experiment. Immedi- 
ately below this, put the following tabular form for the 
readings : 

Observations 



Length of note-book cover . 
Width of note-book cover . 
Metric equivalent of liquid quart 
Capacity of small bottle 
Capacity of tumbler . 
Weight of note-book . 
Metric equivalent of a pound . 



cm. 
cm. 



9- 
9- 



Units of Length. — (a) Examine a meter stick, noting 
its subdivisions. In your laboratory note-book, just below 
the table of observations, rule horizontal lines of the fol- 
lowing lengths, labeling each line with its length : 

1 decimeter, 1.1 decimeters, 1.5 decimeters, 5 centimeters, 
2.5 centimeters, 1.3 centimeters, 1 centimeter. 

(J) Measure in centimeters and tenths of a centimeter 
the length of the cover of your laboratory note-book. 
Similarly measure the width. Record the dimensions. 

Units of Volume and Capacity. — (tf) On a separate 
piece of paper, lay off a diagram like Fig. -4. 



20 



LABORATORY EXERCISES 



1 

1 


I 


1 
1 




1 


I 


1 

J 






I 








*-t 






<~lcm-. — => 


<-\—l cm-r^> 


< — lcm^> 


< — 1 cm^-> 




t 








»-s 








* 







Cut around the diagram with a pair of scissors. Bend 
over the little flaps and fold into a cube, pasting the 
r — ^ flaps on the inside so as to 

hold the cube together. 

The little cube, if accu- 
rately made, is a cubic centi- 
meter, the unit of volume. 
1000 cubic centimeters give 
the liter, the unit of capac- 



Fig. 4. 

ity. For convenience, the 
measuring instruments for 
liquids are usually cylindri- 
cal vessels, marked off in 
cubic centimeters and 
known as graduates. 

(dT) Using a large gradu- 
ate, determine how many cubic centimeters of water are 
needed to fill an ordinary quart measure. 

(To be done in groups of four students unless otherwise directed 
by the instructor.) 

(/) Using a small graduate, find the capacity in cubic 
centimeters of the small bottle furnished you. 

Similarly determine the capacity of an ordinary drink- 
ing tumbler. 

Units of Weight. — The weight of a cubic centimeter 
of water at its maximum density (4° C.) is taken as the 
unit of weight, the gram. 

1000 grams make a kilogram, a weight used for measur- 
ing large quantities. 




Fig. 5. Dissected Liter Block. 



PROPERTIES OF MATERIALS 21 

(f) Using a platform balance, find the weight in grams 
of your laboratory note-book. Record. 

(g*) Determine how many grams are needed to counter- 
balance an ordinary pound weight. Record. 

Tables for the calculated results should be placed at 
the top of the right-hand page of the note-book, and the 
calculations worked out just beneath them. 

Express the number of cubic centimeters found in (c?) 
as the decimal part of a liter. Using this number, calcu- 
late the equivalent of a liter in quarts, carrying the result 
to two decimal places. 

Calculate from the comparison of weights found in (</), 
the equivalent of a kilogram in pounds and tenths of a 
pound. 

Calculated Results 

1 liter qts. 

1 kilogram lbs. 

Discussion : 

In what respects was the convenience of the Metric 
System shown in your measurements? Place the answer 
to this question on the right-hand page of the note-book, 
heading it "Discussion." (Under this heading are to be 
written the answers to any italicized questions occurring 
in the experimental directions.) 



EXPERIMENT 2 

Properties of Materials 

OBJECT. To examine a few common substances so as to deter- 
mine their properties. 

Apparatus. Triangular file ; pocket-knife ; hammer ; anvil, or 
flatiron (with detachable handle). 



22 



LABORATORY EXERCISES 



Material. Copper wire #18, or some larger size; strips of 
sheet lead about 3± n X \ n \ pieces of small glass tubing ; paraf- 
fin ; rubber bands, or strips of sheet rubber ; steel nails. 

Introductory: 

Every substance has its own set of properties. Certain 
of these are the well-marked or characteristic properties 
by which we recognize the substance. These characteristic 
properties are important in that they determine the prac- 
tical use of a substance. 

Experimental: 

The substances to be examined are copper, glass, rubber, 
lead, paraffin, wood, and steel. Take them in any order. 
Tabulate on the left-hand page of your note-book the 
results of your examination, in a table like that given 
below. 



Substance 


Hardness 


Lustre 


Malleability 


Elasticity 


Copper 










Glass 










Rubber 










Lead 










Paraffin 










Wood 










Steel 











PROPERTIES OF MATERIALS 23 

Hardness. — Use a knife blade or a file to determine 
the hardness. Describe this in comparative terms, as 
very soft, soft, somewhat hard, hard, and very hard. 

Lustre. — Note two kinds of lustre or " shine." Which 
substances would be said to be without lustre ? 

Malleability. — Use a hammer, and tap the substance 
on an anvil or other block of iron to ascertain whether or 
not the substance can be hammered out into sheets with- 
out breaking. 

Elasticity. — Try to change the shape of the substance 
by bending. If the substance bends or gives, remove the 
strain to find out whether or not the substance will return 
to its original condition. In determining the elasticity, 
make use of the results obtained in testing for malleability. 

Ductility. — A ductile substance admits of being drawn 
out into fine wire. This property is not easily determined 
in the laboratory by students. Which of the substances 
are ductile ? Why do you think so ? Do not tabulate for 
ductility. 

Write a simple description of how you determined each 
of the properties tabulated. No drawing is necessary for 
this experiment. 

Discussion : 

Under this heading on the right-hand page of- note- 
book, answer any italicized questions occurring in the 
experimental directions, and also the following questions: 
Which of the substances are good conductors of heat? 
Of electricity ? Name any other general properties that 
have not been mentioned in this experiment (Class 
Discussion). 



24 LABORATORY EXERCISES 

EXPERIMENT 3 

Measurement of Bodies 

OBJECT. To find in metric units the volume of a block of wood. 

Apparatus. Wooden block ; metric scale. 

Introductory : 

Iron is " heavier" or more dense than wood. To find 
out how many times as dense, measurements must be made 
of the size and weight of a piece of each. It is more con- 
venient in physical work to make the measurements in the 
metric system, because it is a decimal system. The chief 
units used are the centimeter and the gram. 

Experimental : 

On the left-hand page of your note-book and immedi- 
ately below the statement of the object of the experi- 
ment, put a tabular form like the following for the 
measurements to be made: * 

Observations 

Number of block 

Length of block cm. 

Width of block cm. 

Thickness of block cm. 

When the scale is placed so that the scale divisions touch 
the block, there will be less error in reading measurements. 

1 Note to Instructor. Many teachers find it desirable to have the 
students write in their laboratory note-books, previous to coming into the 
laboratory, the number, the title, and the object of the experiment, and 
any tabular form of measurements to be made. As this will be the first 
experiment in many courses, the directions for the note-book record have 
been made very definite. 



MEASUREMENT OF BODIES 



25 



2 3 



irii|iiii|iin 

5 



I 



Fig. 6. 



The eye must be directly in front of the point on the scale 
and the point located in the block. Why is it desirable 
to estimate to hundredths of a division on a scale divided 
into tenths? 

Using the scale in this 
way, find the length, 
breadth, and thickness of 
the block furnished you. 
Do not make measure- 
ments at bruised corners. 

From your apparatus 
make, on the left-hand page of the note-book, an outline 
drawing similar to that given (Fig. 6). 

On the same page write a brief description of what you 
did, touching on the points regarding measurements which 
you were instructed to observe. Complete in the laboratory 
at least the drawing and the description. The left-hand 
page of the note-book should be finished before the right- 
hand page is begun. 

On the right-hand page, place the table of calculated 
results, the calculations themselves, the answers to the 
questions for discussion, and the formal conclusion. The 
tables of calculated results should always be placed at the 
top of the right-hand page. 



Calculated Result 



Volume of block 



cm. 



In making the calculations for the above results, indicate 
the units of measurement for each result. Do not carry 
out the calculated volume beyond the hundredths of a 
cubic centimeter. Read the discussion on " Significant 
Figures," pages 27-29. 



26 LABORATORY EXERCISES 

Discussion : 

Under this heading on the right-hand page answer any- 
italicized questions occurring in the experimental direc- 
tions. Why would it be desirable to make several meas- 
urements of each dimension of the block and take the 
average for the calculation ? 

Conclusion : 

The volume of block No. . is cm. 3 . 



SIGNIFICANT FIGURES 

Accuracy in Scientific Calculations. — Calculations in scientific 
work are based on readings obtained by some method of meas- 
urement. The calculations cannot be more accurate than the 
figures with which they are made. Yet beginners in physics, 
in their zeal to be accurate, retain figures in their calculations 
far beyond the point justified by the accuracy of the measure- 
ments. The results are not so accurate as they would be 
if certain figures had been discarded in the progress of the 
calculations. The following paragraphs aim to show how 
scientific accuracy may be obtained in the calculations of 
experimental physics. 

Average Readings or Results. — The dimensions of a rectan- 
gular block may be measured with a metric scale graduated in 
centimeters and millimeters. By estimating the tenths of a 
millimeter, the readings may be expressed to the hundredths 
of a centimeter. 

The following readings might be obtained for the length of 
the block as determined along two of its edges : 

A B 

7.45 cm. 7.45 cm. 

7.42 cm. 7.42 cm. 

7.47 cm. 7.47 cm. 



3 )22.34 cm. 3 )22.34 c m. 

7.44 cm. 7.446 cm. 

(Correct scientific average.) (Incorrect scientific average.) 

The second decimal place in these readings represents the 
estimated tenths of a millimeter. In estimating- such small 
quantities, one may readily misjudge not only by one tenth of 

27 



28 LABORATORY EXERCISES 

a millimeter, but even to the extent of two or three tenths. 
Hence the figures expressing tenths of a millimeter are not 
accurate, but are doubtful figures. They are indicated here in 
heavy-face type. 

In column B the average given for the three readings is 
7.446. In this number the second 4 is a doubtful figure, there- 
fore the 6 in the next decimal place beyond must be more than 
doubtful. This figure 6 means nothing in our units of meas- 
urement. 

Some authorities may claim that 7.45 is nearer to the correct 
average in such a case. Mathematically this is so, but it must 
be remembered that one cannot judge accurately between 0.04 
cm. and 0.05 cm. on a scale whose smallest division is 0.1 cm. 
Hence the average of 7.44 in column A may be regarded by 
the painstaking student as correct and reasonable, particularly 
as the divisor is a small number. 

Retention of Significant Figures. — Let us find the volume of 
a rectangular block with the following dimensions : length, 
7.44 cm. ; width, 4.67 cm. ; and height, 2.82 cm. To find the 
area of the base multiply the length by the width, indicating 
the doubtful figures in heavy-face type. 

7.44 
4.67 



5208 

4464 
2976 
34.7448 cm. 2 

In the first partial product, 5208, all the figures are doubt- 
ful, as they were obtained by multiplying by the doubtful 
figure 7; in the second partial product, 4464, the final 4 is 
doubtful because it resulted from a multiplication in which a 
doubtful figure was a factor ; and for the same reason the 6 in 
the third partial product, 2976, is doubtful. 

In the addition of the partial products, figures which are ob- 



SIGNIFICANT FIGURES 29 

tained by adding doubtful figures, are doubtful figures. This 
makes the last four figures doubtful in the total 34.7448. All 
the doubtful figures but the first should be discarded. Then 
the area of the base as justified by the accuracy from measure- 
ments is 34.7 square centimeters. 

To find the cubical contents multiply the area of the base by 
the height : 

34.7 
2.82 
694 
2776 
694 



97.854 cm. 3 

Discarding all the doubtful figures except the first, 97.8 cm. 3 is 
the correct volume of the rectangular block. 

A student who found the cubical contents without discard 
ing any of the doubtful figures would get as a result 97.980336 
cm. 3 . Not only would he have done extra work, but his result 
would not be scientifically accurate. 

A good rule in making calculations is to retain only signifi- 
cant figures. Significant figures include the first doubtful 
figure and the figures preceding it. 



30 



LABORATORY EXERCISES 



EXPERIMENT 4 



Volume Measurement of an Irregular Body 

OBJECT. To find the volume of a body of irregular shape. 

Apparatus. Solid of irregular shape, as a lump of metal, 
brass hook weight (50 or 100 g.), or large-sized lead sinker; 
cylindrical graduate (100 c.c.) ; strong thread, or string. 

Introductory : 

The volume of a body of irregular shape cannot be 
found by measuring a few dimensions and then making a 
simple calculation. A stone dropped into a glass of water 
raises the water level. As the stone and the water can- 
not occupy the same place at the same time, the volume 
of the stone may be found from the increase in volume. 

Experimental : 

Given a lump of metal and a graduated cylinder with 
water in it, devise a way of getting the volume of the 
metal. 




100- 

95 -J 

90-f 
85 -f 
80-| 
75-= 
70 -E 
65 -E 
GO 

— PX 



30=% 






15- 

^"io^ 



1^. 




Fig. 7. 



MEASUREMENT OF AN IRREGULAR BODY 31 

In reading a graduate, place the eye on the level of the 
lowest point of the curved surface and record this as the 
height of the water. As the graduations are cubic centi- 
meters, and as an error of 1 cm. 3 in the volume that we 
are measuring would be a considerable per cent of error, 
therefore, estimate tenths of a cubic centimeter as nearly 
as you can. 

Make the readings indicated by the table of observations 
and record in a similar tabular form near the top of the 
left-hand page of note-book. 

Observations 

Reading before immersing the metal . . . cm. 3 

Heading after immersing the metal .... cm. 3 

Number of lump of metal 

Material of lump 

On the left-hand page of the note-book, make from 
your apparatus, outline drawings similar to Fig. 7, and 
write a simple description of the experimental method 
used. 

Discussion : 

What property of matter makes possible this method 
of finding the volume ? 



Conclusion : 

Volume of lump of metal No. is cm. 

cm. 3 as . ..cm, 3 . 



3 __ 



32 



LABORATORY EXERCISES 



EXPERIMENT 5 



Density 

OBJECT. To determine the density of wood and of metal. 

Apparatus. Block used in Experiment 3 ; lump of metal 
used in Experiment 4 ; spring balance or other balance ; linen 
thread. 

Introductory : 

Iron is heavier than wood and lead is heavier than iron. 
By this we mean that, if we take pieces of the three 
materials of the same size, the lead has the greatest 
weight, and so we conclude there are more pounds per 
cubic foot (or grams per cubic centimeter) of lead than of 
iron or of wood. That is, the lead has the greatest den- 
sity, for density is the mass per unit volume of a sub- 
stance. In the metric system this is written grams per 

cubic centimeter or -^-r. 
cm. 3 

Experimental : 

All that is necessary for the calcu- 
lations is to know the mass and vol- 
ume. The volume of each of the solids 
to be used has already been obtained 
in Experiments 3 and 4. 

The mass of a body is measured by 
its weight. The greater the mass, the 
more a body will stretch a spring from 
which it is hung. The graduations on 
the scale of the spring balance indicate 
the masses that must be hung upon 
Fig. 8. the hook, in order to pull the pointer 



© 







DENSITY 33 

to each division on the scale. The mass of the block may 
be found, then, by hanging it upon a spring balance. 
Read the balance to tenths of the smallest division. 

If a beam or a platform balance is used, read on page 
11 or on page 9 the directions for its use before perform- 
ing this experiment. 

Observations 

Mass of icood g. 

Mass of metal g. 

From your apparatus make, on the left-hand page of the 
note-book, an outline drawing like Fig. 8. On the same 
page write a simple description of what you did. 

Make the calculations and put the results in a table at 
the top of the right-hand page of the note-book. 

Calculated Results 

Volume of block (from Exp. 3) .... cm 3 

Volume of metal (from Exp. 4) .... cm. 3 

Density of wood g. per cm. 3 

Density of metal ( ) g. per cm. 3 

Conclusion : 

The density of wood is 

The density of is 

(name metal) 



34 LABORATORY EXERCISES 

EXPERIMENT 6 

Elasticity — Hooke's Law 

OBJECT. To find the relation between the elongation of a spiral 
spring and the stretching force, provided the elastic limit is not ex- 
ceeded. 

Apparatus. A closely coiled spiral about 10 cm. long and 
1.7 cm. in diameter, made of #20 spring brass wire, with a hook 
and pointer at one end and at the other a straight section for 
hanging or clamping ; stand with pendulum clamp and meter stick 
clamp ; meter stick ; pan for suspension ; metric weights. 1 

Introductory : 

When a steamboat makes its landing, the large hawsers 
tighten as the boat is swung toward the wharf. The 
diameter of the large rope becomes smaller and measure- 
ments would show the length had been stretched. The 
stretching force has changed both the shape and volume 
of the rope. When the the line is cast off again, the rope, 
because it is an elastic body, recovers very nearly its origi- 
nal diameter and length. Sometimes the stretching force 
is so great that the rope snaps because the ultimate strength 
of the rope has been exceeded. 

In materials subjected to stretching forces, as the wire in 
the coil of a spring balance, the change in diameter is very 
slight, but there is considerable lengthening or elongation. 
The question arises whether the elongation proceeds irregu- 
larly or at a uniform rate as the stretching force increases, 
provided the elastic limit of the material is not exceeded. 

1 The spiral coil may be conveniently made by winding the wire around 
a J" pipe. The special pendulum and meter stick clamps may be replaced 
with ordinary laboratory clamps or other attachments. In case weights 
heavier than those specified for the loads are used, a larger size of wire 
should be selected. 



ELASTICITY — HOOKE'S LAW 



35 



Experimental : 

Place the meter stick in a vertical position, 
the weight pan on the hook of the spring 
and attach the pointer just above the 
hook at right angles to the spring. Sus- 
pend the spring so that the end of the 
pointer is close to the metric scale, but 
does not touch it. Also try to adjust the 
position of the spring so that the pointer 
is opposite some main division of the metric 
scale such as the 10-cm. or 20-cm. mark. 
This mark is the zero reading or the 
point from which the first elongation is to 
be measured. Record this zero reading. 

Put a 5-gram weight in the pan and 
read the position of the pointer. Take 
off this weight and allow the spring to 
go back. Again read the position of the 
pointer. Now put on the 10-gram weight 

Observations 



Suspend 




Fig. 9. 
Continue in 



Load on Pax 


Heading of 
Pointer 


Zero Heading 


Corrected Reading 
(Total Elongation) 


5 grams 


cm. 


cm. 


cm. 


10 grams 


cm. 


cm. 


cm. 


15 grams 


cm. 


cm. 


cm. 


20 grams 


cm. 


cm. 


cm. 


25 grams 


cm. 


cm. 


cm. 


30 grams 


cm. 


cm. 


cm. 


35 grams 


cm. 


cm. 


cm. 


40 grams 


cm. 


cm. 


cm. 


45 grams 


cm. 


cm. 


cm. 


50 grams 


cm. 


cm. 


cm. 


55 grams 


cm. 


cm. 


cm. 


60 grams 


cm. 


cm. 


cm. 



36 LABORATORY EXERCISES 

this manner, increasing the load 5 grams at a time and 
recording the results in tabular form near the top of the 
left-hand page. The total elongation due to the load is 
the difference between the pointer reading and the zero 
reading which is made each time. 

Make a drawing from your apparatus, and write a sim- 
ple description of the experimental method. 

Curve on Cross Section Paper. With the loads taken 
and the total elongations obtained, plot a curve on cross 
section paper, placing loads on the perpendicular axis and 
total elongations on the horizontal axis. Attach the cross 
section paper by one edge to the right-hand page of note- 
book. 

Discussion : 

What kind of a curve is obtained ? What relation does 
this show between the total elongation and the stretching 
force ? How elastic should the spring be in order to obtain 
very exact results ? Was your spring such a spring ? 
What is the principle upon which a spring balance works ? 

Conclusion : 

Complete the following statement of Hooke's Law : 
When the elastic limit is not exceeded, the distortion of 

a body due to a stretching force is to the 

force. 



TENACITY OF WIRE 



37 



EXPERIMENT 7 

Tenacity of Wire 

OBJECT. To determine (a) the relation between the tension 
and the elongation of a wire; (b) the comparative tenacity of 
copper, iron, and brass. 

Apparatus. Block for clamping wire ; pulley with stem ; 
thumb tacks ; weight carrier; slotted weights — 1 lb., 2 lb., 2 lb., 
5 lb., <0 lb.; millimeter scale; large-sized needle; magnifier (a 
cheap convex lens may be used). 

Material. Spools of iron, brass, and copper wire, f 28; 
sealing wax. 

Introductory : 

When a load is suspended by means of a cord, the cord 
stretches. As the suspended weight is increased, the cord 
stretches further until it finally breaks. A wire or a metal 
rod behaves in the same way, but the elongation is smaller 
and not so readily noticed. There is, however, definite 
elongation. This must be allowed for in the construction 
of bridges and other structures. By experimenting with 
fine wire under increasing loads, we can follow all the 
changes until the wire breaks. 







(fi^ 



T^fMli.ftnHi.ftufti.VM 



1M^ 




Fig. 10. 



38 



LABORATORY EXERCISES 



Experimental : 

(a) The block is clamped to one end of the laboratory 
table and the stem of the pulley set into a hole bored 
diagonally into the opposite end. 

A piece of wire about 30 cm. longer than the table is cut 
off. This is clamped to the binding post, given a turn around 
the wooden cylinder, and attached to the weight carrier at 
the other end. Care must be taken that there are no kinks 
or sharp bends anywhere in the wire. The wire is then 
placed over the pulley and the needle attached at right an- 
gles to it with a drop of melted wax at a point near the pulley. 

The millimeter scale is then fixed in place beneath the 
the needle with the thumb tacks so that its divisions are 
parallel to the needle. 

A 2-lb. weight is next placed on the carrier to straighten 
the wire ; then it is removed and the zero reading of the 
needle taken, tenths of the smallest scale division being 
estimated. A lens may be used to advantage in estimating 
tenths. 

Weights are now added, a pound at a time, the amount 
of stretching force and the reading of the needle on the 
scale being noted and immediately recorded in tabular 
form near the top of the left-hand page. 

After each reading remove the weights and again note 
the zero reading. The force which causes the first con- 
siderable shifting in the zero point is known as the elastic 
limit. Continue the readings until the wire breaks. 



Observations on 



Wire, Gauge No. 



Stretching Force 


Zero Reading 


Reading of Pointer 


Breaking Strength 


etc. 


etc. 


etc. 


etc. 






TENACITY OF WIRE 39 

(5) Replace the broken wire with another of different 
material, and add the weights one pound at a time until 
the wire breaks, without recording the elongations. Re- 
peat with as many wires as the instructor may designate. 
Record results in tabular form on the second left-hand 

page. 

Observations, Part (b) 



Material of Wire 



GaiTtE Number 



Breaking Strength 



On the left-hand page of the note-book make a simple 
drawing of your apparatus, and write a simple description 
of how the experiment was done. 

On the right-hand page, at the top, place the calculated 
results for Part (a) in tabular form. 

Calculated Results 

Stretching force 1 lb. 2 lb. 3 lb., etc. 

Elongation mm. mm. mm., etc. 

Curve. — On a piece of cross section paper, plot a 
curve, laying off forces as abscissae (horizontal) and 
elongations as ordinates (vertical) to the scale given by 
the instructor. Compare the force at the point where the 
curve begins to turn with the elastic limit. Paste the cross 
section paper by one edge into the note-book. 

Discussion : 

Does the wire follow Hooke's Law in that "the dis- 
tortion (elorgation) is proportional to the stretching 
force," through any part of the test as shown by the 
curve ? If so, up to what point ? 



40 LABORATORY EXERCISES 

Conclusion : 

(1) State the relation between the tension of a wire 
and its elongation (a) up to the elastic limit, (6) beyond 
the elastic limit. 

(2) Arrange the materials tested in the order of their 
tensile strength, placing the strongest first. 



EXPERIMENT 8 

Relation between Pressure and Depth 

OBJECT. — To find the relation between the depth of a sub- 
merged surface and the pressure upon it. 

Apparatus. 1 A test tube loaded with shot, upon which 
melted paraffin has been poured, so that the tube will float 
vertically; a paper centimeter scale, attached vertically to the 
inside of the tube with paraffin; weights — 1 to 10 grams if 
a 6" x f" test tube is used and 5 to 20 grams if a 8" x 1" test 
tube is used ; battery jar or hydrometer jar ; cross section paper. 

Introductory : 

When a stick is thrown endwise into water, it springs 
back into the air. When a boat floats in water, there 
must be an upward pressure of the water on it to balance 
its weight. When more heavily loaded, it sinks more 
deeply, but the upward pressure must then also balance its 
weight. 

Experimental : 

A glass tube loaded so that it will remain upright will 
be floated in a jar of water. A scale on the inside of 
the tube will be used to measure changes in depth. This 

1 The method of this experiment was called to our attention by 
Dr. H. C. Cheston of the High School of Commerce, New York City. 



RELATION BETWEEN PRESSURE AND DEPTH 41 



tube should float freely and should not be allowed to 
touch the sides of the jar. The scale readings are 
taken by sighting through 
the jar along the under side 
of the water surface. By add- 
ing small weights as indicated 
in the table below, the level 
of the bottom of the tube 
may be changed. By compar- 
ing the changes in depth and 
the changees in weight pro- 
ducing them, we may find how 
the upward pressure of the water 
(which balances the weight 
of the tube) varies with the 
depth of the surface on which 
it acts. 

Place your observations in a table near the top of the 
left-hand page. 









r 

^ u 




^7£?£^>5-^^^>7 


I-£I-^£?Z : £:-^.-_2 : -^ 1 £ 


~-EHH2rE^:-=E 


I Z ^H>3^=^X 


:-z-2 -3^-3-3£?-3: 


_-_-z-z--_---_^ 


-.-—^r.—^rj^-—^r— 


j==||=lli=^i= 




j=iHHH=?-:§.: 


---^3-3-^3--^- 


ifH^^fX; 


f 




z-BSSE-^zSz 


^sw^Ai?^^^^ 


rz^rzjzzjrziz^i7^rj^^r^r^^j^tr±=^i 


'-^^^EZ^^^- — IEr35H3 



Fig. 11. 



Number of 
Observation 

1 

2 
3 
4 
5 
6 



Observations 

Weight 



Loaded tube alone .... 
Loaded tube alone + 2 grams 
Loaded tube alone + 4 grams 
Loaded tube alone + 6 grams 
Loaded tube alone + 8 grams 
Loaded tube alone + 10 grams 



Scale 
Reading 

cm. 
cm. 
cm. 
cm. 
cm. 
cm. 



Make a drawing from your apparatus and write a simple 
description of the method of the experiment. 

Make the following tabulations at the top of the right- 
hand page: 



42 LABORATORY EXERCISES 

Calculated Results 

Numbers 

1 — 2 

1 — 3 

1 — 4 

1 — 5 

1—6 



Change of 


Change of 


Pressure 


Depth 


grams . 


cm. 


grams . 


. . cm. 


grams . 


cm. 


grams . 


cm. 


grams 


cm. 



Curve on Cross Section Paper. — The readings of change 
of pressure and change of depth should be plotted on 
cross section paper, depths on the perpendicular axis and 
pressures on the horizontal axis. Use a scale of 5 small 
spaces to 1 gram, and 2 small spaces to 1 mm. If the 
resulting graph is a straight line, we may conclude that 
twice the depth was caused by twice the pressure and so 
on, or that the pressure is directly proportional to the 
depth. Paste the cross section paper by one edge in the 
note-book. 

Discussion : 

At each observation in the experiment, what relation 
must exist between the total weight of the floating tube 
and the upward pressure of the water? Why is it not 
necessary to consider any sidewise pressures that may be 
exerted on the tube ? 

Conclusion : 

What is the relation between the pressure on a sub- 
merged surface and the distance of that surface below the 
surface of the liquid ? 



ARCHIMEDES' PRINCIPLE 43 

EXPERIMENT 9 

Archimedes' Principle 

OBJECT. To determine the relation between the loss of weight 
of a sinking solid and the weight of a liquid displaced by it. 

Apparatus. Lump of coal with thread, or copper wire $22 at- 
tached ; overflow can ; catch bucket or beaker with wire loop for 
suspension ; spring balance (250 g.), or beam balance ; battery jar. 

Introductory : 

It is much easier to lift the anchor of a boat when the 
anchor is in the water than when it is out of the water. 
The displaced water supports part of the weight of the 
anchor, and so makes it seem lighter, because the upward 
pressure of the water on the bottom of the anchor is 
greater than the downward pressure on the top. The 
anchor displaces a volume of water its own size. We wish 
to compare the loss of weight of a body submerged in a 
liquid with the weight of the liquid displaced by it. 
This was first done by Archimedes, and the relation found 
is called Archimedes' Principle. 

Experimental : 

Use a piece of coal for the solid. By weighing it 
in air, with a spring balance, and then when immersed in 
water in a jar, the loss in weight of the lump can be found. 

When a can with a spout, called an overflow can, is filled 
and placed on a level table, the water will run out to the 
level of the spout. By placing a weighed beaker under 
the spout and carefully lowering the coal into the can, 
the water which overflows may be caught and weighed. 
Comparing the weight of this displaced water with the 
loss of weight of the coal, will give the relation sought. 



44 



LABORATORY EXERCISES 



Record the following readings in tabular form near the 
top of the left-hand page : 



Observations 

Weight of coal in air 

Weight of coal in water 

Weight of catch bucket 

Weight of catch bucket + displaced water 



9- 
9- 
9- 
9- 



Briefly describe what you did, illustrating each step 
with a drawing from your apparatus, similar to Fig. 12 
(A, B, and C). 



A 



B 



CD 



T 



Fig. 12. 

Calculated Results 



Loss of weight of coal in water . 
Weight of an equal volume of water 



9- 
9- 



Conclusion : 

State the relation between the loss of weight of a sink- 
ing body and the weight of a liquid displaced by it. 



LAW OF FLOTATION 



45 



EXPERIMENT 10 



Law of Flotation 

OBJECT. — To determine the relation between the weight of a 
floating body and the weight of a liquid displaced by it. 

Apparatus. Block loaded to float upright on water ; overflow 
can ; catch bucket or beaker with wire loop for suspension ; 
spring or beam balance. 

Introductory : 

The cork float on a fishline exerts no pull on the line. 
The weight of an ocean liner is supported by the upward 
push of the water. A boat is said to have a certain num- 
ber of tons displacement, 
depending upon its size and 
weight. What is the rela- 
tion between this number 
of tons of water displaced 
and the weight of the boat ? 



A 



T 




Experimental : 

A method similar to that 
used in Experiment 9 will 
give us the relation between 
the weight of the wooden 
block and the weight of the 
liquid displaced by it. Place the table of observations 
near the top of the left-hand page. 



B 



Fig. 13. 



Observations 

Weight of block 

Weight of catch bucket, empty . 

Weight of catch bucket + displaced water 



9- 
9- 
9* 



46 LABORATORY EXERCISES 

Write a simple description of the steps in the expert 
ment, illustrating each with a drawing from your apparatus. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

Weight of water displaced by floating body . . g. 

. f floating body .... g. 
Comparison of weights \ j. 7 , 

1 J displaced ivater . g. 

Conclusion : 

The weight of a floating body and the weight of the 
liquid displaced by it are . 



EXPERIMENT 10 (Alternative) 

Law of Flotation 

OBJECT. To determine the relation between the weight of a 
floating body and the weight of the liquid displaced by it. 

Apparatus. A wooden bar 20 cm. long and 1 cm. square 
with metric scale attached and loaded so as to be a/most sub- 
merged when floating upright in water; 1 hydrometer jar or 
battery jar ; platform balance ; metric weights. 

Introductory : 

The cork float on a fishline exerts no pull on the line. 
The weight of an ocean liner is supported by the upward 
push of the water. A boat is said to have a certain num- 
ber of tons displacement, depending upon its size and 

1 The ordinary wooden hydrometer can be made available by drilling 
a hole in the lower end, adding lead shot, and closing with a cork plug. 
The weight of the bar should be so adjusted that the bar will float almost 
submerged. Finally put a light coat of paraffin over the end which was 
opened. 



LAW OF FLOTATION 



47 



weight. What is the relation between this number of 
tons of water displaced and the weight of the boat ? 

Experimental : 

The wooden bar is to be weighed and then floated in 
the water of jar so as to note the depth to which 
it is submerged. The metric scale on the bar 
gives the length of the column of water dis- 
placed and, like the bar the column of displaced 
water, is 1 centimeter square. Therefore the 
reading on the metric scale is numerically equal 
to the number of cubic centimeters of displaced 
water. Since a cubic centimeter of water at 
ordinary temperatures weighs approximately a 
gram, the weight of the displaced water can eas- 
ily be found. A comparison of the weight of the 
floating bar and the weight of the displaced 
water will bring out the principle of flotation. 




Fig. 14. 



Observations 



Weight of bar 

Length of column of displaced water 



cm. 



Make a drawing of the floating bar from your apparatus 
and write a simple description of the experimental method. 



Calculated Results 

Volume of water displaced by floating body 
Weight of water displaced by floating body 



Comparison of weights 
Conclusion : 



J floating body 
[ displaced water , 



cm. 

9< 

9 



3 



The weight of a floating body and the weight of the 
liquid displaced by it are . 



48 



LABORATORY EXERCISES 



EXPERIMENT 11 

Specific Gravity of Solids 

OBJECT. To find the specific gravity of various solids. 

Apparatus. Spring balance, or beam balance arranged for 
weighing in water ; battery jar ; pieces of coal, glass, and marble, 
or other solids desired. 

Introductory : 

Lead is a very heavy metal. While a pailful of water 
weighs only about 20 pounds, the weight in pieces of lead 
that would just fill the pail would be about 225 pounds. 
Lead weighs about 11.2 times as much as the same volume 

of water. We say that 



A 



T 



CD 



T 



B the " specific gravity " 

r J^ k of lead is 11.2 times. 

The specific gravity of 
a substance is the num- 
ber of times a piece of 
the substance is as 
heavy as the same vol- 
ume of water. 

Experimental : 

It will be necessary 
to get the weight of a 
lump of coal and the 
Fig. is. weight of the same vol- 

ume of water. The 
weight of the coal can be found directly with a spring 
balance, and Archimedes' Principle will help us in getting 
the weight of an equal volume of water. If the coal is 
weighed while immersed in water, it will weigh less than 



SPECIFIC GRAVITY OF SOLIDS 



49 



in air by an amount equal to the weight of water having 
the same size (volume) as the coal. The specific gravity 
of the other solids furnished may be found in the same 
way. 

Record the weighings in tabular form near the top of 
the left-hand page. 

Observations 



Coal 



Marble 



Glass 



Weight of body in air . 
Weight of body in water 



Then make drawings from your apparatus and write a 
simple description of how the experiment was done. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 



Coal 



Marble 



Glass 



Weight of water size of solid . 

Weight of solid 

Specific gravity of solid . . 



Conclusion : 

The specific gravity of coal is times; the specific 

gravity of marble is times; the specific gravity of 

glass is times. 



50 



LABORATORY EXERCISES 



EXPERIMENT 12 

Specific Gravity of a Liquid 

(Bottle Method) 

OBJECT. To obtain the specific gravity of a solution of copper 
sulphate with a specific gravity bottle. 

Apparatus. Specific gravity bottle ; spring balance (250 g.) 
with scale pan, or beam balance ; bottle or jar of copper sulphate 
solution provided with a siphon delivery tube, ending with rubber 
connection, pinchcock, and glass jet tube (Fig. 17). 

Material. Water ; saturated solution of copper sulphate j 1 small 
cloths for wiping. 



Introductory : 

If we find the weight of a 
gallon of water and of a gallon 
of alcohol, we can directly deter- 
mine the specific gravity of the 
alcohol by finding how many 
times it is as heavy as water. 
This is a general 
method for finding 
the specific gravity 
of any liquid. 



Experimental : 

We use small spe- 
cific gravity bottles 




(rft\ 



i 



Fig. 17. Jar and siphon for 
solution. 



Fig. 16. 

having perforated glass stoppers, as in this way we can 



1 A hot saturated solution should be made and allowed to cool, or a 
cheesecloth bag full of copper sulphate crystals should be suspended in 
the top of a jar of water and allowed to stand at least twenty-four hours, 
or until no more copper sulphate will dissolve. 



SPECIFIC GRAVITY OP A LIQUID 51 

obtain very exactly equal volumes of the two liquids. 
The weight of the specific gravity bottle must first be 
found. Then it is to be weighed full of water and next 
full of copper sulphate solution. By comparing the weight 
of the copper sulphate solution filling the bottle with the 
weight of the water filling the same space, the specific 
gravity of the copper sulphate solution may be found. 

CAUTION. Using the wiping cloths if necessary, see that the 
bottle is dry on the outside before weighing and avoid handling it 
except by the neck, for the heat of the hand is likely to drive out 
some of the liquid through the stopper, after it has been fitted. After 
the water weighed has been emptied out, rinse the bottle with a little 
of the sulphate solution. 

Record the weighings in tabular form near the top of 
the left-hand page. 

Observations 

Weight of scale pan and empty bottle .... g. 
Weight of pan and bottle filled with ivater . . gi 
Weight of pan and bottle filled with copper sul- 
phate solution g. 

Make drawings from your apparatus and write a short 
description of how the experiment was done. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

Weight of water filling bottle g* 

Weight of copper sulphate solution filling bottle . g. 

Specific gravity of copper sulphate solution . . times 

Conclusion : 

The specific gravity of copper sulphate solution is 

times. 



52 LABORATORY EXERCISES 

EXPERIMENT 13 

Specific Gravity of a Liquid 

(Hydrometer Method) 

OBJECT. To find the specific gravity of a copper sulphate solu- 
tion by the hydrometer method. 

Apparatus. Hydrometer jars ; square wooden hydrometer 
graduated in millimeters; glass hydrometer for heavy liquids 
(1 to 2). 

Material. Water ; saturated solution of copper sulphate as 
in Experiment 12. 

Introductory: 

A boat, passing from fresh water into the ocean, rises 
a little, as the boat displaces its own weight in each 
case, and the salt water, being more dense, has less volume 
for the same weight. An electric light bulb in concen- 
trated sulphuric acid floated with 100 c.c. of its volume 
submerged ; in alcohol, which is half as dense as sulphuric 
acid, the same bulb would sink until 200 c.c. were sub- 
merged. We see, then, that the greater the specific gravity 
of a liquid the less portion of a given floating body will be 
submerged in it. More exactly, the volumes of a floating 
body submerged in two liquids are inversely proportional 
to the specific gravities of the two liquids. 

Experimental : 

(a) A graduated float used for obtaining the specific 
gravity of liquids is called an hydrometer. The hydrom- 
eter to be used is a loaded stick 1 cm. square and graduated 
in centimeters and tenths. If we now immerse this in 
water (Fig. 18) and record the depth to which it sinks, 
and then do the same with a copper sulphate solution 



SPECIFIC GRAVITY OF A LIQUID 



53 



(Fig. 19), the hydrometer will sink deeper in the less 
dense liquid. The volume of each liquid displaced may 
be measured by the depth of the submerged part of the 
hydrometer, since each centimeter of length rn^eans 1 c.c. 
of volume. If, then, we divide the length submerged in 
in water by the length submerged in copper sulphate, we 
shall obtain the specific gravity of the copper sulphate 
solution. 



*?■' — ■ - ^ 



Fig. 18. 



<-' ^ 



Fig. 19. 



r* '••• ■; '• 'h* 



Fig. 20. 



(5) Direct-reading hydrometers are made of glass tubes 
loaded so as to float upright and provided with a scale 
which gives the specific gravity directly (Fig. 20). After 
completing calculations on part (a), ask the instructor 
for such a hydrometer, and with it find the specific gravity 
of your solution, as a check on your results. Record the 
observations in tabular form near the top of the left-hand 
page. 

Observations 



Heading of bar in water 

Reading of bar in copper sulphate solution 
Reading of glass hydrometer in copper sulphate 
solution 



cm. 
cm. 



54 LABORATORY EXERCISES 

Make drawings from your apparatus showing the posi- 
tion of the wooden hydrometer in the two liquids and 
the position of the glass hydrometer in the copper sulphate 
solution. Accompany these drawings with a short de- 
scription of the method of work. 

Calculated Result 

Specific gravity of copper sulphate solution 

as determined by wooden hydrometer . . times 

Discussion : 

Explain why the volume of water displaced was divided 
by the volume of copper sulphate solution displaced. 

Conclusion : 

The specific gravity of the copper sulphate solution 

by this method (wooden hydrometer) is times 

by the bottle method (Experiment 12) is times 

by the direct reading of the glass hydrom- 
eter is times 



SPECIFIC GRAVITY OF A LIQUID 55 

EXPERIMENT 14 

Specific Gravity of a Liquid 

(Hare's Method) 

OBJECT. To find the specific gravity of alcohol and of a salt 
solution by Hare's method. 

Apparatus. Two 90 cm. lengths of \" glass tubing ; lead or 
glass T-tube, or Y-tube ; 2 rubber connections ; black rubber 
tubing of length convenient for suction ; screw compressor ; ring 
stand and clamp for supporting T-tube or Y-tube ; 2 tumblers 
(preferably of thin glass and with nearly vertical sides), or 2 
beakers. 

Material. Distilled water, if available ; saturated solution of 
common salt, and grain alcohol in stock bottles provided with 
siphon tubes about -f^" bore. 

Introductory : 

The simple barometer is nothing more than a long tube, 
closed at one end and filled with mercury, which is then 
inverted in a dish of mercury. A mercury column about 
76 centimeters in length remains standing in the tube. 
This column is held up by the pressure of the atmosphere. 
It has also been determined experimentally that the 
pressure of the air supports a much longer column of 
water — approximately 34 feet. We know that mercury, 
volume for volume, is much heavier than water, or, as we 
say, has a greater specific gravity. The fact that the 
atmosphere holds up columns of liquid whose length varies 
with the particular liquid taken, has been utilized in an 
ingenious method for determining the specific gravity of 
liquids. 



56 



LABORATORY EXERCISES 




Experimental : 

The apparatus (Fig. 21) consists of two long parallel 
tubes with their lower ends dipping into tumblers of 

liquids. The upper end of each 
is joined by a rubber connection 
to an arm of a T-tube. To the 
center tube of the T is attached a 
rubber tube to be used for suc- 
tion, which can be closed by a 
screw compressor. 

(a) Half fill one tumbler with 
water and the other with a sat- 
urated solution of salt. 

With the rubber tubing open, 
compare the water levels inside 
and outside the long tube. Ac- 
count for this condition of levels. 
Is it also true for the levels of the 
salt solution ? 

Suck out a little air through 
the rubber tube, noting the be- 
havior of the liquids. What pres- 
sure causes the liquids to rise in the 
tubes ? 

Again remove air by suction 
until the water column is pushed 
up nearly to the top of its tube. 
Pinch the rubber tube tightly and 
close the screw compressor. Note 
the relative height of the two liq- 
uids. The pressure on the upper surfaces of the two 
liquids is the same. How does this pressure compare with 
the outside air pressure ? What pressure forced the liquids 




Fig. 21. 



SPECIFIC GRAVITY OF A LIQUID 57 

up into the tubes? How does this pressure compare with the 
downward pressure of each liquid? Compare, then, the 
downward pressure of the water column with that of the salt 
solution. 

Measure with a meter stick the length of the water 
column above the level of the water in the tumbler. 
Obtain similarly the length of the column of the salt 
solution. Record the measurements in tabular form near 
the top of the left-hand page. 

(5) Open the compressor and allow the liquids to run 
back into their tumblers. Return the salt solution to its 
stock bottle and rinse out the tumbler. Detach the long 
tube used for the salt solution, and, after washing, attach 
it again. 

Put grain alcohol into the empty tumbler and repeat 
the experiment so as to obtain the length of the water 
and the alcohol columns, taking care not to suck the alcohol 
up into the mouth. Tabulate the measurements near the 
top of the left-hand page. 

Return the alcohol to its stock bottle. 

Observations 
Part (a) ; 

Length of the water column . . . . . . cm. 

Length of the salt solution column .... cm. 

Part (6) ; 

Length of the water column . . . . . . cm. 

Length of the grain alcohol column .... cm. 

Make an outline drawing of the apparatus used, and 
write a simple description of the general method of the 
experiment. 

With the water and the salt solution, the downward 
pressure per square centimeter of each, balances the same 
amount of atmospheric pressure. The two columns must 



58 LABORATORY EXERCISES 

then have the same weight. Being of equal cross section, 
their lengths are proportional to their volumes. But the 
greater the specific gravity of a liquid, the smaller the 
volume for a given weight. Are the relative weights, 
then, directly or inversely proportional to the heights of the 
columns? With this relation in mind, calculate the spe- 
cific gravity of the salt solution and of the alcohol, relative 
to water. Record the results in tabular form at the top 
of the right-hand page. 

Calculated Results 

Specific gravity of the salt solution . . = times 

Specific gravity of the alcohol .... = times 

Discussion : 

Answer under this heading on the right-hand page the 
italicized questions occurring in the directions. 

Conclusion : 

The specific gravity of the salt solution is times ; 

the specific gravity of the alcohol is times. 



EXPERIMENT 14 (Alternative) 

Specific Gravity of Liquids 

(Balancing Columns) 

OBJECT. To find the specific gravity of (a) carbon tetrachloride, 
(b) grain alcohol, by the method of balancing columns in a U-tube. 

Apparatus. 2 Mohr burettes (50 c.c.) connected by a piece 
of thick- walled rubber tubing of sufficient length ; H of mann screw 
compressor ; ring stand ; two burette clamps ; 2 glass funnels. 
2±", or tops of two thistle tubes ; beaker; medicine dropper. 



SPECIFIC GRAVITY OF LIQUIDS 59 

Materials. Mercury ; distilled water if available ; carbon 
tetrachloride ; grain alcohol. (Other liquids, such as glycerine 
kerosene, etc., as the instructor desires.) 

Introductory : 

When mercury fills the lower rounded portion of a U- 
tube, the mercury stands at the same level in the two 
arms, since the downward pressure of the air is the same 
on the two mercury surfaces. 

When a certain volume of water is poured into one arm 
of this same tube, and an equal volume of kerosene into 
the other arm, the mercury level in the water arm is 
lower than that in the kerosene arm. Since the mercury 
is free to move, the given volume of water must press 
down with greater weight on the mercury than does the 
same volume of kerosene. Accordingly, volume for 
volume, the kerosene weighs less than the water. Usually 
the specific gravity is found by calculating the ratio be- 
tween weights of equal volumes. Since this is so, might 
not the inverse ratio between the volumes of equal weights 
give the specific gravity ? 

Experimental : 

As we have seen, equal weights may be measured by 
the downward pressure of liquids. The equal weights 
can be obtained by pouring just enough of each liquid 
into its arm of the U-tube, so as to make the two mercury 
surfaces stand at the same level. All that remains is the 
measurement of the volumes of the two liquids and the 
finding of the ratio, remembering that it is an inverse 
one. 

Clamp the two burettes at about a third of their length 
from their lower ends and in a vertical parallel position 
with the 50-c.c. marks horizontally opposite each other. 



60 



LABORATORY EXERCISES 



Slip the screw compressor over the rubber connecting 
tube and attach the ends of the tube to the burettes. 

Pour mercury through a thistle 
tube top or funnel at the top of one 
burette until the mercury surface 
in each burette stands at the 50-c.c. 
graduation, or some mark a short 
distance above (Fig. 22). Squeeze 
out the air bubbles in^the connect- 
ing tube before taking the zero 
reading of the mercury levels. 

(a) Record the zero reading of 
the burettes in the table of obser- 
vations. Then close the screw com- 
pressor on the connecting tube. 

Into the right-hand burette pour 
enough carbon tetrachloride to half 
fill the burette. Add about the same 
volume of water to the other burette. 
Cautiously open the compressor a lit- 
tle, noting whether the tetrachloride 
column is balanced by the water. 
If not, close the compressor, add 
more water, and test again. Con- 
tinue in this manner until the water 
balances the tetrachloride, as shown 
by the mercury remaining at the 
same levels when the compressor is 
opened wide. A medicine dropper 
is convenient for adding the last 
portions of water needed. 
Read and record the top levels of the balancing columns. 
Raise the tetrachloride burette so that the mercury just 
runs into the connecting tube. Over this end of the tube 




Fig. 22. 



SPECIFIC GRAVITY OF LIQUIDS 61 

close the screw compressor and slip off the rubber tube, so 
that the tetrachloride can empty into a beaker placed below 
the burette. Pour the tetrachloride into its stock bottle. 

(5) Rinse out the open burette with a few cubic centi- 
meters of alcohol (or other liquid to be used) and again 
connect the rubber tube. 

Then obtain as in (a) a column of alcohol which 
balances the water column in the left-hand burette. 

Record all readings in a tabular form near the top of the 
left-hand page. 

Observations 
Part (a) : 

Reading of mercury levels . ...... cm. 3 

Heading at top of water column cm. s 

Reading at top of tetrachloride column . . . cm. 3 

Part (6) : 

Reading of mercury levels . cm. 3 

Reading at top of water column cm. 3 

Reading at top of alcohol column cm. 3 

Make an outline drawing of your apparatus and de- 
scribe briefly how the experiment was done. 

Place the table of calculated results at the top of the 
right-hand page. The specific gravities are to be calcu- 
lated with reference to water. 

Calculated Results 

Part (a) : 

Volume of the water column cm. 3 

Volume of tetrachloride column cm. 3 

Specific gravity of tetrachloride . . = times 

Part (6) : 

Volume of water column cm. 3 

Volume of alcohol column cm. 3 

Specific gravity of alcohol .... = times 



62 LABORATORY EXERCISES 

Discussion : 

Why is the specific gravity in this experiment the in- 
verse ratio of the volumes of the balancing columns ? 

Conclusion : 

The specific gravity of carbon tetrachloride is 

times. The specific gravity of alcohol is times. 



EXPERIMENT 15 

Density of Air 

OBJECT. To determine the approximate density of air in the 
room. 

Apparatus. Air pump; round-bottom flask (250 c.c.) with a 
tightly fitting 1-hole rubber stopper carrying a glass inlet tube 
with a piece of thick-walled rubber tubing attached; screw 
compressor ; beam or horn pan balance weighing to 0.01 gram ; 
metric weights ; graduate ; large battery jar, or pail. 

Introductory : 

It is very evident that lead has weight. Even a small 
child knows that a tumbler of water is heavier than the 
empty glass. We know that solids and liquids have 
weight, but does the air which surrounds us have weight ? 
If balloons are lighter than air, the air must have weight. 
It would be interesting to find out just how dense air is, 
that is, the number of grams to a cubic centimeter. 

Experimental : 

A flask may be weighed full of air and then the air 
partially pumped out. Then the exhausted flask may be 
weighed. The difference between the two weights is the 
weight of air pumped out of the flask. The volume of 



DENSITY OF AIR 



63 





Fig. 23. 



this air may be found by measuring the water which will 
run into the exhausted flask. With the 
weight and volume of the air known, the 
density (grams per cubic centimeter) may 
be found. 

Make all weighings to the nearest cen- 
tigram. In all weighings of the flask, in- 
clude the rubber stopper with its tubing 
and screw compressor, and any wire sus- 
pension used with the balance. See that 
all joints between rubber and glass are 
tight before exhaustion. Allow T at least 
five minutes for the exhaustion of the 

flask, and be sure 
the screw compres- 
sor is tightly closed 
before the removal 
of the rubber tube from the pump. 
Immerse most of the flask in 
water and open the screw com- 
pressor a little at a time under 
water. As soon as no more water 
will run in, move the flask so that 
the level of the water on the in- 
side is the same as that on the 
outside (Fig. 24). 

Pinch the rubber tube with 
the compressor so as to close it, 
and remove the flask from the 
water. Set it in a secure upright 
position on the table. Open the 
compressor so as to allow the water 
in the small tube to run down into the flask and then re- 
move the stopper and its connections. 



Fig. 24. 



64 LABORATORY EXERCISES 

Measure with a graduate the volume of water in the 
flask. 

Record the measurements in tabular form near the top 
of the left-hand page. 

Observations 

Weight of flask filled with air g. 

Weight of flask, air exhausted g. 

Volume of air exhausted cmfi 

Record, if so directed by the instructor, the temperature 
of the room and the barometric pressure. 

Briefly describe the steps in the experiment, illustrat- 
ing with drawings from your apparatus. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

Weight of air exhausted g. 

Volume of air exhausted cm. 3 

Density of air grams 

cm. 3 
Discussion : 

After the water had run into the flask, the water levels 
were made the same, so that any air not pumped out of 
the flask would be at the same pressure as the air in the 
room. What is the necessity for this precaution? Would 
the results obtained for this experiment be exactly the 
same on different days ? Give reasons for your answer. 

Conclusion : 

The density of the air in the laboratory at the existing 
conditions was grams per cubic centimeter. 



DENSITY OF AIR 65 



EXPERIMENT 15 (Alternative) 

Density of Air 

OBJECT. To determine the approximate density of air in the 
room. 

Apparatus. Incandescent lamp bulb; Bunsen burner ; blow- 
pipe ; small battery jar ; small funnel and graduate ; horn pan 
balance weighing to 0.01 gram or better; metric weights; small 
squares of adhesive plaster. 1 

Introductory : 

It is very evident that lead has weight. Even a small 
child knows that a tumbler of water is heavier than the 
empty glass. We know that solids and liquids have 
weight, but does the air which surrounds us have weight ? 
If balloons are lighter than air, then air must have weight. 
It would be interesting to ascertain just how dense air is, 
that is, the number of grams to a cubic centimeter. 

Experimental : 

The bulb of an incandescent lamp is empty save for 
the filament and a very slight trace of gas which was not 
exhausted. The bulb then can be weighed empty. By 
making a small hole, the air will rush in and fill the bulb. 
Another weighing gives the weight of the bulb filled with 
air. The difference between the two weighings is the 
weight of the air in the bulb. The volume of this air 
may be found by filling the bulb with water and then 
measuring the water with a graduate. With the weight 

1 Note to Instructor. If the supply of burnt-out bulbs is limited, the 
experiment may be done in small squads, each student making the 
weighings and measurements for himself. In small classes the instructor 
may prefer to make the first air hole with the blowpipe. 



66 



LABORATORY EXERCISES 




Fig. 25. 



and volume of the air known, the number of grams per 

cubic centimeter can be calculated. 

Filling the Bulb with Air. — Use the tiny point of a 

blowpipe flame, but approach the portion to be heated 
very gradually with the flame so as to 
avoid the sudden cracking and collapsing 
of the bulb. Heat a small area near the 
top of the bulb where the diameter is 
greatest (Fig. 25). As the glass softens 
at the tip of the blowpipe flame, the pres- 
sure of the outside air will make a hole. 
Any bits of glass which may be chipped 
off will tend to be drawn inward, so that 

there will be no loss of weight due to the glass. Only a 

tiny hole is needed to admit the air. 

Filling the Bulb with Water. — After the bulb has 

been weighed full of air, heat it with the tip of a blow- 
pipe flame so as to make a little hole in the glass an inch 

or so from the base of the lamp. 

When the heated glass is cool, immerse the bulb upright 

in the water of a battery jar so as to leave the first air hole 

made just above the surface of the water 

(Fig. 26). When the bulb is nearly full, 

incline the bulb, so that the rest of the 

space can fill with water. 

Then take the small square of adhesive 

plaster and stick over the lower hole, 

holding it in position for a couple of 

minutes with the finger. Now cover the 

upper air hole with the finger and remove 

the bulb from the water. Holding the 

bulb nearly upright over a funnel sup- 
ported in a graduate, pierce through the adhesive plaster 

just over the lower air hole. When the finger over the 




Fig. 26. 



DENSITY OF AIR 67 

upper air hole is removed, the water will run down into 
the funnel. Remember that the outward flow may be 
stopped at any time by closing the upper hole with the 
finger. 

Record the measurements in tabular form near the top 
of the left-hand page. 

Observations 

Weight of incandescent bulb empty .... g. 

Weight of bulb filled with air g. 

Volume of air filling bulb cm. s 

Record, if so directed, the temperature of the air in the 
room and the barometric pressure. 

Describe briefly the steps in the experiment and illus- 
trate with drawings from your apparatus. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

Weight of air filling bulb g. 

Volume of air filling bidb . cm. s 

Approximate density of air , ^ — — - 

Conclusion : 

The approximate density of air in room at existing con- 
ditions was grams per cubic centimeter. 



68 LABORATORY EXERCISES 

EXPERIMENT 16 

Boyle's Law 

OBJECT. To find how the volume of a gas varies with the pres- 
sure exerted upon it. 

Apparatus. Barometer ; Boyle's Law apparatus as furnished 
by dealers in scientific instruments. The two forms recommended 
are : ( 1 ) the apparatus with the closed tube ending in glass stop- 
cock, and the open tube connected with the closed tube by heavy- 
walled tubing; (2) the apparatus with both tubes dipping into a 
mercury reservoir, the closed tube sealed at the upper end, and 
a small bicycle pump to produce pressure in reservoir, so as to 
make mercury rise in the two tubes. 1 

Material. Mercury, if not supplied with the apparatus. 

Introductory : 

A bicycle pump takes in air and makes it occupy a much 
smaller space. We know that the air in the inflated tube 
is under much greater pressure than before. Oxygen is 
sold in steel cylinders filled under pressure. When the 
valve is opened, many jars of oxygen may be obtained from 
one tank for experiments in the chemical laboratory. 
The total volume of the jars filled is far greater than that 
of the cylinder, for the oxygen is under much less pressure 
in the jars than in the steel tank. The two instances of 

1 Note to Instructor. The directions for this experiment have been 
written so that either of the two forms of apparatus may be used. Both 
forms are on hand in many schools. A good type of the first apparatus 
may be obtained from the C. H. Stoelting Co., Chicago (list number 1151); 
the second form with an improved mercury reservoir is made by the 
L. E. Knott Apparatus Co., Boston (list number 41-105). 

The authors regard the J-tube form as very desirable for demonstration 
purposes, but less fit for the laboratory experiment, as most students are 
unable to handle it without spilling the mercury required. 



BOYLE'S LAW 69 

the inflated tire and the filling of jars with oxygen show 
, that there is some relation between the volume of the gas 
and the pressure exerted on it. Whether or not there 
is any regularity in this relation, may be ascertained 
by experiment. 

Experimental : 

Specific directions for handling the apparatus will be 
given by the instructor. 

The volume of air used is that inclosed above the 
mercury in the closed tube. The mercury in the open 
tube is used for varying the pressure upon the inclosed 
air. When the mercury levels are the same in the two 
tubes, the inclosed air is under atmospheric pressure. 
When the mercury level is higher in the open tube, 
then the inclosed air is under more than atmospheric 
pressure, for a column of mercury equal in height to the 
difference in levels is adding its pressure to the atmospheric 
pressure. A lower level in the open tube means a pressure 
less than the atmospheric. 

The pressure is expressed in centimeters of mercury. 
If the bore of the closed tube is of uniform diameter, the 
length of the inclosed air column may be taken as the 
measure of its volume and recorded in centimeters. 

Make a number of readings, as directed by the in- 
structor. The difference of the mercury levels in the 
open tube between successive readings, should be about 
10 cm. One reading should be made with the mercury at 
the same level in the two tubes. 

As soon as the readings are made, record them in tabu- 
lar form at the top of the left-hand page. 

Write a simple description of the method of using the 
apparatus and make an outline drawing of it, showing the 
essential parts. 



70 



LABORATORY EXERCISES 



Observations 



Number of 


Column of Inclosed Air 


Mercury Level 


. Keading 


Top 


Bottom 


Open Tube 


1 

2 
etc. 


cm. 
cm. 


cm. 
cm. 


cm. 

cm. 



Barometric pressure at on was mm. = cm. 

A (time) (date) 

Place the calculated results in tabular form at the top 
of the right-hand page. The difference in the mercury 
levels can be found from the quantities in the last two 
columns of the table of observations. 

The pressure of the inclosed air is atmospheric pressure 
plus or minus (as the case may be) the difference of 
mercury levels. In recording the product of the pressure 
by the volume, omit the decimal fractions. 

Calculated Results 



Number of 


Difference 


Pressure of 


Volume of 


Pressure X 


Heading 


in Levels 


Inclosed Air 


Inclosed Air 


Volume 


1 


cm. 


cm. 


cm. 3 




2 


cm. 


cm. 


cm. 3 




etc. 











Discussion : 

Is the product of the pressure and the volume approxi- 
mately constant ? Why should the temperature of the 
inclosed air not change while the readings are being made? 
Would a variation in the barometric pressure during the 
experiment affect the result ? 

Conclusion : 

Complete the following statement : 

At a constant temperature, the volume of a given mass 
of gas varies as the pressure sustained by it. 



MEASUREMENT OF GAS PRESSURE 71 

EXPERIMENT 17 

Measurement of Gas Pressure 

OBJECT. To measure the pressure of the laboratory gas supply. 

Apparatus. Water manometer, consisting of a U-tube (8") 
with one arm carrying a tightly fitting 1-hole rubber stopper with 
glass elbow tube ; x block with slot or groove for supporting U-tube ; 
foot rule or a metric scale ; rubber tubing for connecting ma- 
nometer with gas cock ; barometer. 

Introductory : 

The bag of a balloon connected with a gas main, fills 
and rounds out as the gas rushes in. One can feel the 
gas pressing out when a stopcock is opened from the gas 
supply in the laboratory. The balloon fills and the gas 
rushes into the room despite the fact that the weight of 
the air is pressing around the bag of the balloon and 
against the opening of the gas cock. This pressure, which 
is effective against the atmospheric pressure, may be de- 
scribed as the effective pressure of the gas supply. How 
much is the effective pressure of the gas delivered to our 
homes and school ? 

Experimental : 

Enough water is added to the U-tube to fill it about 
halfway up, and then the stopper carrying the elbow tube 
is pressed tightly into one arm of the tube. The water 
levels in the two arms are at the same height, since the 
air presses down on both water surfaces equally. 

The elbow tube is connected by a rubber tubing with 

1 Instead of the U-tube, a U-shaped "bend of glass tuhing with the arms 
about 8 n long, may be used. A Skidmore stand is very convenient for 
supporting the U-tube. 



72 



LABORATORY EXERCISES 



the gas supply. The gas stopcock is slowly turned on 
and the difference in the height of the water levels meas- 
ured. This measurement should be made as soon as the 
rising water level reaches its greatest height. 



Observations 

Atmospheric pressure (barometer reading} . 
Difference in height of water levels . 
Time ivhen readings were made .... 



in. 
in. 



If the measurements were made in centimeters, change 
them to inches by multiplying by 0.3937. 

Write a simple description 
of the experiment and make 
a drawing showing how your 
apparatus indicated the gas 
aas — *■ ^ \ ls[-illl pressure. 

The difference of the water 
I ^ levels due to the increased 

pressure is independent of the 
cross section of the U-tube, 
therefore we can consider its 
cross section to be 1 square 
inch. A pressure of 14.7 
pounds to the square inch 
holds up a water column 
33.57 feet in length. From 
this equivalent, calculate the 
pressure in pounds per square 
inch of a column of water 
equal. in height to the differ- 
ence of levels measured in 
the U-tube. This will give the effective pressure of the gas. 
A pressure of 14.7 pounds to the square inch holds up 



o ^=j 




MEASUREMENT OF GAS PRESSURE 73 

a mercury column 30 inches in length. From this rela- 
tion, calculate the pressure in pounds per square inch 
which is equivalent to the observed barometric reading. 

Adding the effective pressure to the atmospheric pres- 
sure gives the total pressure of the gas, that is, the pressure 
per square inch within the gas pipes. 

Record the calculated results in a table at the top of 
the right-hand page. 

Calculated Results 

Effective pressure of gas per sq. in lb. 

Atmospheric pressure per sq. in lb. 

Total pressure of gas per sq. in. ..... lb. 

Discussion : 

Why is it not necessary to remove the air in the arm 
of the U-tube connected with the gas supply? What is 
the gas pressure stated to be in your town or city ? What 
does this mean ? 

Conclusion : 

The effective pressure of the gas in the laboratory at 
on , was pound per square inch. The total 

(time) (date) 

pressure per square inch in the gas pipes was pounds. 



74 LABORATORY EXERCISES 

EXPERIMENT 18 

Water Pumps 

OBJECT. To study the parts and the operation of the simple 
lift pump and the force pump. 

Apparatus. Glass models of a lift pump and a force pump ; 
3 feet of glass tubing (i' f ) with a short piece of rubber tubing 
attached ; battery jar. 

Introductory : 

The ordinary suction or lift pump has been used for over 
two thousand years. Although both the lift pump and 
the force pump are articles of familiar appearance, few 
can give an intelligent explanation of their operation. 
In these cases, as in other apparently simple devices, the 
study of the principles upon which they are based proves 
fascinating. 

Experimental : 

CAUTION. Handle the glass models with great care. Do not 
spill water around the laboratory. 

(a) Place in a jar of water the lower end of a long 
glass tube which has a short rubber tube on the upper 
end. Compare the water levels in the tube and in the 
jar. Account for the relative levels. 

Suck out through the rubber tube most of the air in the 
glass tube, noting the action of the water. Pinch tightly 
the upper end of the rubber tube. Does the water run 
back ? What pressure holds up the column of water in the 
glass tube? Release the pressure on the rubber tube. 
What happens? Explain. Wliy is it necessary to remove 
some of the air in a tube if we want ivater to be pressed up 
in it? 



WATER PUMPS 



75 



Make three simple diagrams which will show 
what was done in this part of the experiment 
and indicate the results. 

(b) The Lift Pump. — Examine a glass model 
of a lift pump, noting the suction tube, the bar- 
rel, the piston, the two valves, and the spout. 
Make an outline drawing, labeling the parts. 
Starting without any water in the pump, im- 
merse the suction tube in a jar of water and 
operate the pump till it is in full action, noting 
the action of the inclosed air, the water, and the 
two valves on each successive stroke. Record 
the observations in tabular form on the left-hand 
page. What is the main thing accomplished by 
the first few strokes of the pump ? 



f 1 



Fig. 28. 



Observations on the Lift Pump 



Stroke 


Valye 


Action of Air 


Action of Water 


Action and 
Use of Yalye 


1st Up 


Lower 








1st Up 


Upper 








1st Down 


Lower 








1st Down 


Upper 








2d Up 


Lower 








2d Up 


Upper 








etc. 


etc. 









(c) By a rubber connection attach a long glass tube 
to the suction pipe of the lift pump. Dip the free end of 
the long tube into a jar of water placed on the laboratory 
floor. Can you pump water from the floor? What limits 
the vertical distance through which water can be taken by a 
lift pump even though it were mechanically perfect ? 



76 



LABORATORY EXERCISES 



(d) The Force Furnp. — Examine the glass model of a 
force pump, noting its parts. Try its action. 

Make two diagrams showing the action 
of the pump — one for the up stroke, the 
other for the down. Show water levels, 
and use arrows to indicate the direction 
of water flow. 

Will the force pump or the lift pump 
raise water to a higher level ? Why is 
this so? 

Do not write a description of the work 
done, as the drawings and tabulations 
show this. A few explanatory statements 
may be added if necessary. 

Discussion : 




Fig. 29. 



Under this heading, on the right-hand 
page, answer the italicized questions in the 
experimental directions. 
Is the action of these pumps due to pressure or to "suc- 
tion." Which type of pump is a bicycle pump? Explain. 
Why is a little water sometimes poured in at the top 
of a pump just before working the handle? (Class Dis- 
cussion.) 






THE PRINCIPLE OF MOMENTS 77 

EXPERIMENT 19 

The Principle of Moments 

OBJECT. When three parallel forces are in equilibrium, to com- 
pare (a) the forces in one direction with the force in the opposite 
direction; (b) the clockwise moments with the counterclockwise 
moments. 

Apparatus. Meter stick ; loops of strong cord ; 3 spring balances 
(2000 grams), with hooks for suspending them, or clamps for 
fastening the balances to the edge of the table top (Fig. 35 ). 1 

Introductory : 

When a team of horses is drawing a wagon, their com- 
bined force forward is exerted to overcome the resistance 
of the wagon pulling backward. When two boys carry a 
heavy weight suspended from a stick, the boys pull up- 
ward and the weight pulls downward. If the boys have 
not equal strength, the weight will be shifted toward one 
of the boys. Which one? 

In each of these cases, we have three forces parallel to 
each other, two in one direction and one in the other. 
These forces are in equilibrium when the stick is balanced. 
If one boy should lift more than he had been lifting, the 
stick would turn toward him. The turning effect of a 
force is called the moment of the force. 

We can imitate either of these cases by attaching three 
spring balances to a meter stick, so that two pull in one 
direction and one in the other. We can then compare 
(a) the pull of the two forces in one direction with that 

1 This experiment can also be conveniently done by using two balances 
suspended vertically with a weight between, supported by a loop on the 
meter stick so that the weight may be moved to positions of equilibrium. 
If this modification is made, allowances must be made for the pull on the 
balances due to the weight of the meter stick. 



78 



LABORATORY EXERCISES 



of the single force in the other, and compare (6) the turn- 
ing effect or moment of the force at one end with that 
of the force at the other end of the stick. 

Experimental : 

The apparatus will be arranged as shown in the diagram 
(Fig. 30). The amount of each force may be read on the 
balance. First each outside cord should be placed 10 cm. 

from its end of the meter 
stick and the third cord 
in the center. See that 
all cords are parallel. 
The highest reading on 
any balance should not 
be more than 1600 grams. 
When all is adjusted, the 
reading of each balance 
and the position of each 
string on the meter stick 
should be recorded (I). 
One end balance may then be shifted so that it is half 
as far from the center as the other. After adjustment, 
readings should again be taken (II). The total force in 
one direction may then be compared with the total force 
in the other, as indicated in the table for the right-hand 
page. The moment of a force is found by multiplying 
the force by its lever arm. The lever arm is the perpen- 
dicular distance from the fulcrum about which the force is 
trying to turn the body, to the force. In this experiment, 
the distance between each of the outer cords and the center 
cord will be the lever arm for the force applied by the 
cord, if the cords are at right angles to the meter stick. 
The moment of each of the end forces around the center 
cord is to be computed. 




Fig. 30. 



THE PRINCIPLE OF MOMENTS 79 

Record the readings in tabular form near the top of the 
left-hand page. 

Observations 

i n 

Reading of balance A . 

Reading of balance B . 
Reading of balance C . 
Point of application of force A 
Point of application of force B 
Point of application of force C . 

Make a drawing of your apparatus and write a simple 
description of how it was used . Place the table of calcu- 
lated results at the top of the right-hand page. 

Calculated Results 

i ii 

Combined Force of A and B 

Force of O 

Moment of A about C 

Moment of B about (7. 

Discussion : 

Is the moment of A about C clockwise or counter- 
clockwise ? Is the moment of B about clockwise or 
counterclockwise ? 

Conclusion : 

Complete the following with a statement about the 
amount of force in each direction : 

When three parallel forces act on the same body to pro- 
duce equilibrium, then 

Complete the following by comparing with the moment 
of the third force around the second, both as to magnitude 
and direction : 

When three parallel forces act on the same body to 
produce equilibrium, the moment of one of them about 
the second is 



80 



LABORATORY EXERCISES 



EXPERIMENT 20 

The Lever Arm of a Force 

OBJECT. To determine the lever arms of non-parallel forces. 

Apparatus. Meter stick, with a hole on the center division 
near one edge, drilled slightly larger than the shank of a -f" screw 
eye; short piece of board about |/' stock; screw eye, ■ ¥ "; fish 
line; four clamps; half meter stick; draughtsman's triangle, 
90°, 60°, and 30.° 

Introductory : 

In using such a lever as a crowbar, pump handle, or 
hammer, it is seldom that the forces exerted on and by the 
lever are parallel to one another. Under such circum- 
stances, it would be desirable to know whether the lever 
arm is to be measured along the lever or at right angles to 
the applied force. 



fiH^i 




Fig. 31. 




THE LEVER ARM OF A FORCE 81 

Experimental : 

The meter stick is to be attached by the screw eye to a 
short board held firmly by two clamps to the edge of the 
laboratory table. The 
meter stick must be free 
to rotate around the shank 
of the screw eye as a ful- 
crum. 

The hook of each bal- 
ance is to be attached by a 
loop to the meter stick. 
The other end of each bal- 
ance is to be clamped to Fi 32 
the edge of the table op- 
posite the meter stick. These two balances are to be 
clamped so that they make acute angles with the meter 
stick and, if possible, these angles should be different, as 
shown in Fig. 31. 

Perpendicular distances may be measured by using a 
triangle and a half meter stick, as shown in Fig. 32. 

Make the following readings and record in tabular form 
near the top of the left-hand page of note-book. 

Observations 

Reading of balance A g, 

Heading of balance B ....... g 

Point of application of force A cm 

Point of application of force B cm 

Position of fulcrum on meter stick .... cm 

Perpendicular distance, fulcrum to force A . cm 

Perpendicular distance, fulcrum to force B . cm 

Make one drawing showing the arrangement of your 
apparatus and another drawing showing the method of 



82 LABORATORY EXERCISES 

measuring the perpendicular distance of a force from the 
fulcrum. Write a simple description of how the experi- 
ment was done, referring to the drawings. Place the table 
of calculated results at the top of the right-hand page and 
make all the calculations on that page. 

Calculated Results 

Distance along stick from fulcrum to a . . . cm. 

Distance along stick from fulcrum to b . . . cm. 

Force A x meter stick distance from fulcrum . 
Force B x meter stick distance from fulcrum . 
Force A x perpendicular distance from fulcrum 
Force B x perpendicular distance from fulcrum 

Discussion : 

Which pair of products, in the table above, more nearly 
agrees with the principle of moments ? 

Conclusion: 

How should the lever arm of a force always be measured? 



EXPERIMENT 21 

Composition of Several Parallel Forces 

OBJECT. When a number of parallel forces are in equilibrium, 
to compare (a) the forces in one direction with the forces in the 
opposite direction ; (b) the clockwise moments with the counter- 
clockwise moments. 

Apparatus. Meter stick ; four or more spring balances 
(2000 g.), with cords and clamps. 



COMPOSITION OF SEVERAL PARALLEL FORCES 83 

Introductory : 

A floor or bridge beam is frequently supported at more 
than two points and has a number of different persons or 
objects exerting their weights on it at various points. It 
is interesting to determine whether the principle of 
moments which has been tested for two forces acting 
about the point of application of a third as a fulcrum, 
will apply to this case also. 

Experimental : 

Four or more spring balances, as the instructor may 
direct, are to be attached by cords to a meter stick, as in 



850 g 



-25.2- 



-20.9- 



1000 g 



-65J 



/JOOgr 



•F 



12-* 
|T50{7 ' 



-30- 



Scalelcm=5Q0g 



1800 g 
Fig. 33. 

the experiment on the Principle of Moments (see Fig. 30, 
page 78). The balances should then be strained and 
clamped in place in such a way as to make all the cords 
parallel, and at right angles to the meter stick. 

The amounts of various forces and their lever arms are 
to be recorded near the top of the left-hand page in the 
form of a diagram like that shown in Fig. 33. Letter 
the forces in order from left to right. 



84 



LABORATORY EXERCISES 



Take for the center of moments some point which is 
not the point of application of any of the forces. The 
line representing each force should be drawn to a scale to 
be designated by the instructor and the exact amount of 
the force should be noted at the right of the line repre- 
senting it. The lever arms are indicated by dimension 
lines as shown. No drawing of the apparatus will be 
necessary. A short description, however, of the experi- 
mental method should be written. 

Place a table like the following at the top of the right- 
hand page and make all calculations on that page: 



Calculated Results 



Clockwise Moments 


COUNTEKCLOCKWISE MOMENTS 


Moment of A 

etc 

Total clockwise moments . 


Moment of B 

etc . 

Total counterclockwise 

moments 



Sum of forces as A, C, E, etc. 
Sum of forces as B, D, etc. 



Conclusion : 

Fill in the blanks in the following statement so that it 
agrees with your results: 

When a number of parallel forces act on a body, it is 

in equilibrium when the of the forces in one direction 

equals the of the forces in the other direction, and 

the total moments equal the total moments 

about any point taken as fulcrum. 



FOUR FORCES AT RIGHT ANGLES 85 

EXPERIMENT 22 

Four Forces at Right Angles 

OBJECT. When four forces at right angles in one plane produce 
equilibrium, to compare (a) the force in any one direction with the 
force in the opposite direction ; (b) the clockwise moments with the 
counterclockwise moments. 

Apparatus. Composition-of-force board with under side rest- 
ing on four steel balls or marbles ; four pegs ; four spring balances 
(2000 g.) with cords and clamps ; meter stick or other metric ruler. 

Introductory : 

Four boys of different ages might pull on the four sides 
of a piece of burlap so as to stretch it parallel to the top 
of a barrel of vegetables while their father finished the 
heading by putting on a hoop. Each boy probably took 
hold of the burlap at the center of his side, but one or 
more of them soon found it advisable to move his hands 
to one side or the other of the center, so as to prevent the 
burlap from being drawn out of his hands. When the 
burlap was properly stretched, four pulls or forces were 
acting at right angles in one plane. Did the principle of 
moments come to the aid of the smaller boys in the 
family so that they could do their share of the stretching? 

Experimental : 

The hook of each spring balance is to be attached by a 
cord to a peg on the composition-of-force board. The 
pegs should be arranged so that no two of them will be in 
the same row of holes across the board in either direction. 
The other end of each spring balance is to be securely 
clamped (see Fig. 35 on page 89) so that both the cords 
holding it are parallel to a row of holes (Fig. 34). This 



86 



LABORATORY EXERCISES 






O O <> O O O O 

o o o o m o o 
00*0000 
00000 

O O O O O (I 

00000 
• 0000 



-€\K 



X) 



latter figure shows the method of attachment of the bal- 
ances to the board, but not the correct location of the pegs. 
The strain on each balance should be at least 500 grams, 
g and the board, which is 

free to move on its roller 
bearings, should be 
brought to rest by the 
equilibrium of the four 
forces at right angles 
pulling on it. 

The amounts of the va- 
rious forces and their 
lever arms are to be re- 
corded in the form of a 
diagram on the left-hand 
page. Draw, in about 
the middle of this page, a square, 7 centimeters on a side, 
and divide each side into centimeter divisions, and lightly 
rule such cross lines as will locate the positions of the four 
pegs or points of application of the several forces. 

Take for the center of moments some point which is 
not the point of application of any of the forces. To a scale 
designated by the instructor, draw a line representing the 
direction and the exact amount of each force. Indicate 
the amount of each force by figures placed to the right of 
the line representing it. The lever arm of each force is to 
be indicated by a dimension line as in Fig. 33, on page 83. 

Calculated Results 



Fig. 34. 



Clockwise Moments 


Counterclockwise Moments 


Moment of 

etc 

Total clockwise moments . 


Moment of 

. etc 

Total counterclockwise 

moments 



PARALLELOGRAM OF FORCES 87 

Unless the instructor so directs, make no drawing of 
the apparatus. A short description of the experimental 
method, however, should be written. 

Place a table, like the one on page 86, at the top of the 
right-hand page and make all calculations on that page. 

Conclusion : 

State, when four forces at right angles in one plane pro- 
duce equilibrium : 

(«) the relation of the force in one direction to the force 
in the opposite direction ; 

(6) relation of the clockwise .moments to the counter- 
clockwise moments about any point taken as a 
fulcrum. 



EXPERIMENT 23 

Parallelogram of Forces 

OBJECT. To find the relation between three forces acting on a 
body at a point, in order that they may be in equilibrium. 

Apparatus. 3 spring balances (2000 g.) ; fish line or other 
light, strong cord ; 3 Stone clamps or other means of hold- 
ing balances in place ; 30 cm. ruler. 

Note. — Pencils used in this exercise should be hard, with long, 
sharp points. 

Introductory : 

If two boys were to kick a football, one east and the 
other north, at the same instant, the ball would not go in 
either direction, but would take a course somewhere be- 
tween north and east. The general direction that it would 
take would depend upon which force were greater. To 
prevent the football from moving, it would be necessary 



88 LABORATORY EXERCISES 

to apply a third force which should have the proper direc- 
tion and amount to just neutralize the other two. We 
wish to find the relation between three forces at an angle 
to each other, acting on a body at a point in such a way 
as to keep the body at rest. With the football it would 
be possible for a single force to be substituted for the 
forces applied by the two boys. Such an imaginary force 
is known as a resultant force, and the two forces which it 
replaces are component forces. The single force that 
would keep the ball from moving is called the equilibrant 
force. Our problem is to rind {a} how the resultant force 
is related to the component forces in direction and magni- 
tude : (^M how the resultant force is related to the equili- 
brant force. 

Experimental: 

Connect the three spring balances by three cords that 
meet at a point A. Fasten these balances in place 
by clamping the attached wires. Pull on the third balance 
until the pointer on one of the balances is near the end of 
the scale and then clamp the third balance in place. 

Place the right-hand page of the note-book under the 
cords with the center of the page under the point A. 
Mark two points directly beneath each cord. Remove the 
book and through each pair of points draw a line which 
represents in direction the force. Xote and record on the 
diagram, the reading of each balance, calling the balances 
/>. c\ and 7>. Measure from A along each line a distance 
to represent the magnitude of the force, using a scale of 
.1. to 250 grams. Place at the end oi each line an 
►whead to show the direction of the force. 

Select one force as the equilibrant and layoff from A 
the resultant eqtial and opposite to the equilibrant. On 
the two lines representing the components, erect a parallel- 



PARALLELOGRAM OF FORCES 



89 



ogram and draw the diagonal from A. Determine the 
magnitude of the force which this diagonal would repre- 
sent. Compare it with the resultant which you laid off 
and drew. 

Mark on the drawing the lengths of the lines and the 
readings of the balances. No table of results is necessary 




Fig. 35. 

on the left-hand page, but write a simple description of 
the method of the experiment. The drawing has already 
been placed on the right-hand page. 

On the second right-hand page place the table of calcu- 
lated results. 



Calculated Results 



Magnitude of resultant .... 
Magnitude represented by diagonal 

Discussion: 



9- 



(1) What single force would alone produce the same 
effect as the two forces represented by the sides of the 



90 LABORATORY EXERCISES 

parallelogram? (2) Compare the resultant and the diag- 
onal of the parallelogram in direction and in magnitude. 

Conclusion : 

Three forces are in equilibrium when the of two of 

them is in magnitude and in direction to the 



EXPERIMENT 24 

Resolution of Forces 

OBJECT. Given the resultant of two forces and one of the forces, 
to find the other force. 

Apparatus. 2 spring balances (2000 g.) ; 500-gram weight ; 
fish line ; upright, with ring for cord and notch for boom ; light 
hard-wood boom, about 25 cm. long, with a brad in the end. 

Introductory : , 

When a load is hanging from the boom of a derrick, its 
weight is sustained jointly by the tension of the rope sup- 
porting the end of the boom and the outward thrust of 
the boom. These two forces may then be considered as 
the component forces, whose resultant balances the weight 
of the load. If we know the pull on the cord supporting 
the boom and the weight of the load, we can calculate 
the thrust of the boom outward. 

Experimental : 

(a) The apparatus is to be set up as shown in Fig. 36. 
The boom should be horizontal, and when ithas been made so, 
a turn of the cord around the brad in the end of the boom 
will keep it from slipping. When all adjustments have 



RESOLUTION OP FORCES 



91 




—iT- Q /T.i|.l « li.l..l . 



d 



Fig. 36. 



been made, hold the note-book with the right-hand page 
against the boom, and indicate the direction of the farces 
by dots under the 
cords and a line 
drawn along the 
top of the boom. 
Place a dot at the 
end of the boom, 
immediately under 
the brad. Leave the 
apparatus undis- 
turbed while per- 
forming the oper- 

ations of part (6). 

(b) Replace the 
note-book on the table. From the dot marking the com- 
mon point of application of the forces, draw lines through 
the dots that were placed under the cords. From the 
common point of application, continue outward some dis- 
tance the line drawn along the boom. Lay off on the 
line representing the tension, a distance corresponding to 
the reading of the balance, using a scale of 100 grams to 
the centimeter. Mark the end of the measured distance 
with an arrowhead, indicating the direction of the force. 
Do the same on the line representing the weight. Mark 
beside each line the exact number of grams represented. 

The weight is the equilibrant of the tension of the 
cord and the outward push or thrust of the boom against 
the cord. Therefore draw a line upward from the point 
of application equal in length to the line representing the 
weight. With this line as a diagonal and the line repre- 
senting the tension as one side, complete a parallelogram 
having a side extending outward from the point of applica- 
tion, as a continuation of the line drawn along the boom. 



92 LABORATORY EXERCISES 

This side will represent the thrust in direction and magni- 
tude. From the length of this side, the outward thrust 
of the boom may be calculated, using the scale employed 
in laying off the other lines. 

(e) Hook a second spring balance between the cord 
and the boom and pull horizontally until the boom just 
slips out of the notch in the upright. Read the balance 
at this point and record below the drawing on the right- 
hand page : 

Force required to pull out boom .... g. 

Since action and reaction are equal, the inward compo- 
nent of the stretched cord on the boom must equal the out- 
ward thrust of the boom on the cord. 

Make a simple sketch of your apparatus and write a 
brief description referring to the sketch. 

Discussion : 

May the resultant of two forces ever be less than one of 
them ? 

Is a rope that is just strong enough to lift a weight 
vertically, strong enough to lift that weight by means of a 
horizontal boom derrick ? 

Conclusion : 

Given the resultant of two component forces and one of 
the components, state how the other component may be 
found. 



FORCE AT THE CENTER OF GRAVITY 93 
EXPERIMENT 25 

Force at the Center of Gravity of a Body 

OBJECT. To find what is the gravitational force acting at the 
center of gravity of a body. 

Apparatus. Half meter stick loaded at one end; 1 ruler or 
other fulcrum properly supported (see Fig. 37) ; 200-gram weight 
with loop of cord attached; spring balance, or platform balance ; 
metric weights. 

Introductory : 

When we shut a heavy door, we push near the outside 
of the door and not near the hinge. A small boy can 
balance a large boy on a seesaw, by sitting farther out 
on the board. When a body is to be turned about an 
axis, the turning power depends upon how much force is 
exerted and how far from the axis the force is exerted. 
The turning power of a force is called the moment of that 
force and is measured by the product of the force and its 
distance from the axis. The moment of the small boy on 
the seesaw is equal to the moment of the large boy. 1 f 
we know the moment of the large boy and the distance 
of the small boy from the fulcrum, we can calculate what 
the small boy weighs. If both boys get off, the board 
can be balanced so it will not touch at either end. The 
point at which a body must be balanced in order to sup- 
port it is called the center of gravity of the body. 

Experimental : 

The body will be a half meter stick loaded at one end. 
This is first to be balanced over a fulcrum in order to find 

lr The loading may be done by attaching a strip of brass, iron, or 
lead to one end of the half meter stick, at right angles to the stick. 



94 



LABORATORY EXERCISES 



the center of gravity (Fig. 37, A). Then a 200-gram 
weight will be hung about 10 cm. from the free end of the 
bar and the bar again balanced. 

By measuring the distance of the 200-gram weight from 
the fulcrum and multiplying this distance by the weight 
(200 g.), the moment of the 200-gram weight is obtained. 



4|0 




Fig. 37. 

This moment equals the moment of the force at the center 
of gravity about the fulcrum. Then the force at the 
center of gravity is calculated. 

A second trial should be made with the weight at some 
other point on the stick, as 20 cm. from the end. 

Finally the loaded stick is weighed. 

All observations as soon as made should be recorded in 
tabular form near the top of the left-hand page. 

Observations 

Position of center of gravity of loaded l 2 

stick . . 

Position of 2QQ-g. weight 

Position of fulcrum for equilibrium . ._^__. 

Weight of loaded stick 



FORCE AT THE CENTER OF GRAVITY 95 

Make drawings showing how your apparatus was used 
and write a simple description of how the experiment was 
done. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

1 2 

Distance of weight from fulcrum (Z> x ) 

Distance of center of gravity from 

fulcrum (D 2 ) 

Moment of weight about fulcrum 

(200x2)!) 

Moment of force at center of gravity 

Calculated force at center of gravity 



Discussion : 

Define moment of force. Explain the calculation of the 
moment of the force at the center of gravity and the cal- 
culation of the amount of this force. 

Conclusion : 

What gravitational force acts at the center of gravity of 
a body. (Compare the last item in both tables.) 



96 LABORATORY EXERCISES 

EXPERIMENT 26 

The Pendulum 

OBJECT. To observe the effect on the number of vibrations of 
a pendulum in one minute of (a) change in mass, (b) change in 
amplitude, (c) change in length. 

Apparatus. A wood and a metal ball each about 1 inch in 
diameter and having a light cord about 125 cm. long attached; 
a support consisting of a split cork in a burette clamp, or a special 
pendulum clamp, so placed that the pendulum may swing freely 
in front of the laboratory table ; metronome or laboratory clock 
with telegraph sounder. 

Note. — Some instructors prefer to have all pendulums in the 
room released at a given signal and stopped on signal at the end of 
the minute, as confusion is thereby lessened and the student's mind 
is concentrated on the counting. 

Introductory : 

When a clock goes too fast, should the pendulum be 
shortened or lengthened ?. We see pendulums made of 
different materials. Does this affect the length of their 
beats ? Does it take a pendulum longer to swing through 
a long arc than a small one ? These are some of the ques- 
tions the experiment will help to answer. By a vibration 
of a pendulum is meant a swing from one end of its arc 
to the other. The period of the pendulum is the time 
that one vibration takes. A seconds pendulum is one that 
swings from one end of the arc to the other in just one 
second ; a half seconds pendulum makes one vibration in 
one half second ; etc. The frequency of the pendulum is 
the number of vibrations per minute. 

Experimental : 

There will be furnished a metal and a wooden ball 
of the same size, attached to a light cord over a meter 



THE PENDULUM 



97 



I 



long. As the suspending cord is very light, we neglect 
its weight and consider the length of the pendulum as 
the distance from the lower edge of the support to th6 
center of the suspended ball or "bob." 

For the first test, adjust 
the length of the pendulum 
with the wooden ball to 100 
cm. Count and record the 
number of vibrations made 
in one minute swinging 
through a small arc. Re- 
place with the metal pen- 
dulum and find how many 

vibrations that makes in one / J$- 

minute swinging through | ~T~ % | 4, 

the same arc. Comparing 
these numbers will show 
whether or not the material 
of the pendulum affects the 
period of vibration. 

Now swing the metal bob Fig. 38. 

through an arc twice as 

great as before, counting the number of vibrations per 
minute. Make the length of the pendulum 50 cm. and 
find the number of vibrations per minute. Repeat with 
lengths of 36 cm. and 25 cm. 

Record all observations in tabular form near the top of 
the left-hand page. 

Observations 

Vibrations per minute, bob wood, length 100 cm., arc 
small 

Vibrations per minute, bob metal, length 100 cm., arc 
small 



98 



LABORATORY EXERCISES 



Vibrations per minute, bob metal, length 100 cm., arc 

large 

Vibrations per minute, bob metal, length 50 cm., arc 

small 

Vibrations per minute, bob metal, length 36 cm., arc 

small 

Vibrations per minute, bob metal, length 25 cm., arc 

small 



Make a drawing of your apparatus and describe briefly 
how the experiment was done. 

Place the table of calculated results at the top of the 
right-hand page and directly below make all the calcula- 
tions called for. 

Calculated Results 



Length 

loo cm. 
. r )0 cm. 
))(5 cm. 
25 cm. 



Conclusion: 



Nil mi;i:i: OF VIBRATIONS 



Period 



Square of Period 



(a) Does the mass of the pendulum affect the period ? 
(ft) Does the amplitude (if comparatively small) affect 
the period ? (c) Is there any simple relation between the 
period and the length? between the square of the period 
and the length ? 



THE INCLINED PLANE 99 

EXPERIMENT 27 

The Inclined Plane 

OBJECT, (a) To compare the work done in raising a load by- 
means of an inclined plane and in raising it vertically; (b)to 
determine the mechanical advantage from the length and height of 
the plane. 

Note. — Only the case when the force is parallel to the plane is con- 
sidered in this experiment. 

Apparatus. Inclined plane properly supported ; car with cord 
attached ; 500-gram weight or other load ; spring balance (2000 g.), 

Introductory : 

Safe movers roll a safe into a wagon along a sloping 
plank. Does this require less force than to lift the safe 
directly into the wagon ? Is less work done by rolling it 
up the incline than by lifting it directly ? The plank is 
an example of the use of the inclined plane. We wish to 
answer the above questions by using a car on an inclined 
board in the laboratory. We also wish to find out the 
mechanical advantage of the plane. This is the number 
which is obtained by dividing the resistance by the effort. 
In the inclined plane the mechanical advantage may be 
found also from the dimensions of the plane. We shall 
seek to find what dimensions are used and what division is 
made to obtain the mechanical advantage. 

Experimental : 

An iron car loaded with a 500-gram weight will be used 
and it is to be pulled up an inclined plane by means of a 
cord attached to a spring balance. This balance thus 
measures the force employed to draw the car up the plane. 



100 



LABORATORY EXERCISES 



The combined weight of the car and its load is the weight 
lifted by the use of the plane. It may be found with the 
spring balance. The dimensions of the plane are to be 
measured, as shown in Fig. 39. 

Correction is to be made for some friction. This may 
be eliminated by averaging the reading of the balance 
when the car is moving uniformly up the incline with the 




Fig. 39. 

reading when it is moving uniformly down the plane. 
Decide in each case whether the friction is a help or a hin- 
drance. The work done along the plane is measured by 
the product of the balance reading and the length of the 
plane (to A). The work done in raising the weight an 
equal distance is measured by the product of the weight 
lifted and the height of the plane (at .A). 

Record the observations in tabular form near the top of 
the left-hand page. 



Observations 



Weight of car and load . 
Force required, car ascending 
Force required, car descending 
Length of plane .... 
Height of plane .... 



9- 

ff- 

cm. 

cm. 






THE INCLINED PLANE 101 

Make a simple sketch of your apparatus and write a 
short description of the method of the experiment. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

Average force used c g. 

Work = weight lifted x height of plane . . . g.cm. 

Work = force x length of plane g>cm. 

Mechanical advantage = ^ — ..... 

force 

Length of plane 
Height of plane 

Conclusion : 

(a) Compare work done in lifting the load vertically 
from the table to the level of A, with the work done in 
raising it the same vertical distance by rolling it along the 
plane. (5) What relation between the height and length 
of the plane equals the mechanical advantage ? 



102 LABORATORY EXERCISES 



EXPERIMENT 28 

Pulleys 

OBJECT. To study the operation of pulleys and to find their 
mechanical advantage. 

Apparatus. 1 single fixed pulley and 1 double fixed pulley 
with stems for clamping or attaching ; single movable pulley ; an 
additional movable pulley or a movable double pulley with hooks 
for suspending pan or weights ; support for fixed pulley ; balance 
pan 1 ; metric weights ; spring balance (250 g.) ; meter stick ; light, 
strong flexible cord (fish line). 

Introductory : 

The block and tackle is a familiar sight in large cities, 
as it is used for moving pianos and safes in and out of high 
buildings. In the country it is used for pulling stumps 
and handling logs. On the water front, the pulley in 
some form or combination is employed for loading the 
heaviest articles of the cargo. 

Pulleys would not be so widely used unless they 
brought some mechanical gain to their users. The me- 
chanical advantage of a machine may rest in changing 
either the direction or the magnitude of the force applied 
to it. Wherein lies the gain when pulleys are used? 

1 The balance pan for Part {a) is made by first finding with a sensitive 
spring balance the error in indicated weight arising from the use of the 
balance tested in an inverted position. The pan is made from thin sheet 
copper and holes punched in the corners for the fine copper wire used as 
suspension cords. The weight of the pan and its suspension should equal 
the weight error found for the balance. It can be adjusted by filing or 
punching. 



PULLEYS 103 



Experimental : 



(a) The Fixed Pulley \ A spring balance should be used 
with the hook downward, as the weights of the hook and 
the drawbar were acting on the spring when 
the mark for the zero point was located. In ^ , , m 



an inverted position the balance will not s X 



W 



read correctly. To compensate for the error 
arising in this manner, in this experiment, 
the balance pan with its supporting cords has 
been made equal in weight to the drawbar 
and hook. 

The apparatus should be arranged as in 
Fig. 40. A weight is placed in the pan and 
the spring balance is pulled vertically down- 
ward so as to raise the load at a steady rate, p . 40 
the force or effort necessary being read at 
the same time on the spring balance. Then the balance 
reading is again taken as the load descends at a uniform 
rate. The friction increases the balance reading as the 
load ascends and decreases the reading for the load de- 
scending. An average of the two readings may be con- 
sidered as the force or effort which will just equal the 
resistance to be overcome before the load will move. 

Take readings with 100 grams and 200 grams as the 
loads, and record in tabular form. Note the distance 
through which the load is raised as compared with the 
distance through which the effort moves. Compare the 
load with the effort. What is the only mechanical gain in 
using a single fixed pulley ? 

(b) Single Movable Pulley. The apparatus is arranged 
as in Fig. 41. The total load in this case includes the 
weight of the pan and the weight of the pulley block. 
These are weighed separately and the weights recorded. 



104 



LABORATORY EXERCISES 



Readings are made with the 100-gram and the 200-gram 
weights as in (a). How does the distance through which 
total load (resistance) moves compare 
with the effort distance ? What is the 






Fig. 41. 



Fig. 42. 



Fig. 43. 



mechanical advantage of a single movable pulley ? What is 
sacrificed to gain this ? 

(c) Combinations of Pulleys. — A single fixed and a 
single movable pulley are arranged as in Fig. 42. This 
is the arrangement used in the movable scaffolds of 
house painters. Only one set of readings is made — that 
with a load of 200 grams. What additional advantage 
does this combination of pulleys have over the single movable 
pulley ? 

Next, two fixed pulleys (a double pulley) and a single 
movable pulley are combined by the proper adjustment of 
cords. Readings are taken with the 200- and the 500-gram 
weights. The vertical distance through which the load 
moves from the table top is carefully measured as is 
also the distance covered by the effort at the same time. 
Note also the number of cords which support the movable 
block. 

Then a fixed pulley is combined with two movable pulleys 



PULLEYS 



105 



(or a double pulley) and a similar set of readings taken 
with weights of 200 and 500 grams. 

Make for (a), (S), and (e) simple diagrams showing the 
arrangement of the load, the pulleys, and the spring balance. 
Indicate clearly the number of cords which support the 
movable pulley blocks. 

Write simple descriptions of the work done in each 
part of the experiment, shortening the descriptions by 
references to the diagrams. 



Observations 





Pulleys used 


Weights of 


Balance Heading 


Trials 


Load 


Pan 


Movable 
Block 


Up 


Down 


1 and 2 

3 and 4 

5 and 6 

7 and 8 

9 and 10 

11 and 12 

13 and 14 

etc. 


1 fixed 

1 fixed 
1 movable 
1 movable 

1 fixed and 1 mov. 

2 fixed and 1 mov. 
2 fixed and 1 mov- 

etc. 


100 g. 
200 g. 
100 g. 
200 g. 
200 g. 
200 g. 
500 g. 
etc. 











For Part (c) only; Trials 11 to 18 



Number of 


Resistance 
(Total Load) 


Effort 

(Average Balance) 


Distance moved Through 


Trials 


Resistance 


Effort 


11 and 12 










13 and 14 










15 and 16 










17 and 18 











106 



LABORATORY EXERCISES 



Except in Part (a), the total load (resistance) is the 
sum of the weights on the pan, the weight of the pan, and 
the weights of the movable blocks used. The average of 
the two balance readings in each trial is the effort. The 
mechanical advantage of a machine is defined as the re- 
sistance divided by the effort. Record these calculated 
results in a table at the top of the right-hand page. 

Calculated Results 



Trials 



Pulleys 
used 



Eesistance {B) 
(Total Load) 



Effort (E) 
(Average 
Balances) 



Mechanical 

advantage 

R + E 



Cords supporting 
Movable Block 



Discussion: 

Under this heading on the right-hand page (or the 
second right-hand page) answer the italicized questions 
occurring in the experimental directions. - 

Conclusion: 

After comparing in each case the number representing 
the mechanical advantage with the number of cords sup- 
porting the movable block or blocks, answer the following 
question : 

How may the mechanical advantage of a set of pulleys 
be stated in terms of the machine's construction ? 



THE WHEEL AND AXLE 107 

EXPERIMENT 29 

The Wheel and Axle 

OBJECT. To study the operation of the wheel and axle and to 
find its mechanical efficiency. 

Apparatus. Wheel and axle with several diameters ; metric 
weights (500 g. and 1000 g.) ; spring balance (2000 g.) in case 
apparatus has not an exact simple ratio ; fish line ; stand and 
clamp for wheel and axle in case it is not mounted on its own 
base ; pair of calipers (or a pencil compass) is convenient for 
measuring the radii ; meter stick. 

Introductory : 

The windlass is used to lift a bucket from a well or 
dirt from an excavation. Several men on a capstan can 
pull out of the water a heavy anchor which they could not 
lift with their hands from the deck of the vessel. The 
devices for accomplishing these rather difficult tasks are 
applications of the wheel and axle, one of the simple 
machines. In the illustrations just given, a lesser effort 
overcomes a larger resistance, or there is a mechanical ad- 
vantage greater than one. Upon what does the mechani- 
cal advantage of a wheel and axle depend ? 

Experimental : 

One cord is attached to the axle and another cord to 
the wheel. On the axle cord is attached the load (resist- 
ance*) ; on the wheel cord are attached weights which act 
as the effort and just balance the load. When the weights 
on the two cords are in equilibrium, the slightest pull on 
the cord in either direction should make the weights run 
freely up and down at a gentle rate. 

The weights may be attached by a slip noose in the 



108 



LABORATORY EXERCISES 



free end of the cord. The first load may be conveniently 

1000 grams. The distances traveled by the effort and 
the resistance in the same time are measured 
with a meter stick. The radius of the axle 
and the radius of the wheel are also deter- 
mined. All these measurements are to be 
recorded in tabular form near the top of 
the left-hand page. 

At the direction of the instructor, meas- 
urements with additional loads are made. 
In case there are several wheels on the axle, 
one of the smaller wheels may be taken for 
a new axle. For some of the measurements 

it may prove necessary to use a spring balance in place 

of the effort weight. 




Fig. 44. 



Observations 



Number of 
Trial 



Load on Axle 
(Resistance) 



Effort on 
Wheel 



Radius of 
Axle 



Radius of 
Wheel 



1 

2 

etc. 



IOOOj 



etc. 



For Two Readings Only 



Number of 


Load 
(Resistance) 


Effort 


Distance Moved Through 


Trial 


Resistance 


Effort 













Make a drawing of the wheel and axle used and write a 
simple description of how the experiment was done. 



THE WHEEL AND AXLE 



109 



The mechanical advantage of a simple machine like the 
wheel and axle, is the ratio of the resistance to the effort. 
Calculate this for each trial. Also find in each case the 
ratio of the radius of the wheel to the radius of the axle. 
Place all the calculated results in tabular form at the top 
of the right-hand page. 

Calculated Results 



Number of 
Trial 


Resistance (R) 
(Load) 


Effort (E) 


Mechanical 
Advantage R -z- E 


Kadius Wheel 
Radius Axle 













Discussion : 

What is sacrificed in gaining the mechanical advantage 
of the wheel and axle ? 



Conclusion : 

Complete the following statement: The mechanical 
advantage of the wheel and axle may be stated in terms 

of its construction as the ratio of the 

to the 



110 LABORATORY EXERCISES 

EXPERIMENT 30 

Mechanical Efficiency of Machines 

OBJECT. — To find the mechanical efficiency of an inclined plane, 
a set of pulleys, and a wheel and axle. 

Apparatus. As designated for the inclined plane (page 99), 
for the pulley (page 102), and for the wheel and axle (page 107). 

In the experiments on those machines, measurements were 
made and tabulated which will serve for this experiment. 

Commercial block and tackle with necessary weights in case 
Part (b) is to be done. 

Introductory : 

The rapid growth of the manufacturing industries in 
the United States has been due in large part to the develop- 
ment of efficient machinery. To be efficient, a machine 
must return, in some form of useful output, a large part of 
the energy applied to it. Machines which waste too much 
of the applied energy in friction, in loss of motion, or in 
other ways, are condemned to the scrap heap when a 
more efficient machine for the same purpose is devised. 
Calculations of the efficiency of complicated machinery 
are difficult even for a competent mechanical engineer, 
but a student can learn from the inclined plane, the pulley, 
and the wheel and axle, the main factors in the efficiency 
of any machine. These factors are in accordance with 
the law of work, — " the amount of work put into a perfect 
machine equals the work gotten out of it." 

The mechanical efficiency of a machine is the percentage 
of total work done on the machine which proves useful. 

Experimental : 

(a) The instructor may direct the use of the readings 
obtained in the experiments on the inclined plane, the 



MECHANICAL EFFICIENCY OF MACHINES 111 

pulley, or the wheel and axle. In all cases, the effort 
readings used must be ones taken while the weight (re- 
sistance) is being raised, without correction for friction. 
These are the conditions under which a machine does use- 
ful work. 

The weight raised, the height of the plane, the force 
with load ascending, and the length of the plane are the 
readings to be taken from the inclined plane experiment. 

It should be noted with regard to the inclined plane 
that the load (resistance) moves through a useful distance 
equal to the height of the plane while the effort is moving 
the length of the plane. The effort is the force used with 
the load ascending. 

In the pulley and the wheel and axle experiments, most 
of the readings necessary for this experiment were tabulated 
in the second table of observations. The effort reading to 
be taken from the pulley experiment is not the " average 
balance," but the balance reading with the load ascending, 
recorded in the first table of observations. 

The observations taken from previous experiments 
should be again tabulated near the top of the left-hand 
page used for this experiment. Any new observations 
made at the direction of the instructor may be tabulated 
in the same form. 

(5) During the laboratory hour, if the instructor so 
directs, a test will be made on the efficiency of a commer- 
cial block and tackle with as large a load as is safe and 
desirable. The students designated by the instructor to 
make the test will report the results to the class. Com- 
parison can then be made between the school apparatus, 
designed to show the law of work, and commercial appa- 
ratus, made to stand the wear and tear of actual service. 

In a perfect machine, the amount of work obtained 
from it equals the amount of work put into it, i.e. resist- 



112 



LABORATORY EXERCISES 



ance x resistance distance == effort x effort distance. 
Calculate these two products for each observation. 

Then calculate the mechanical efficiency of each machine 
from the two products, recalling that 

Fffi ■ _ useful work (work output) 

^ " total work (work input) 

Observations 



Machine 


Kesistance 
(Load or Weight lifted) 


Effort 

(Force applied) 


Distance moved through 


Eesistance 


Effort 













At the top of the right-hand page tabulate the results 
of all calculations. 

Calculated Results 



Machine 



Useful Work 

(Resistance X 

Resistance Distance) 



Total Work 

(Effort X 

Effort Distance) 



Mech. Efficency 
/ Useful Work\ 
\ Total Work / 



Discussion : 

What may make the mechanical efficiency vary in dif- 
ferent observations of the same machine ? 

Conclusion : 

The average mechanical efficiency found from my ob- 
servations was for the inclined plane %, for the 

pulleys %, and for the wheel and axle %. 

(state combination used) 



COEFFICIENT OF FRICTION 113 

EXPERIMENT 31 

Coefficient of Friction 

OBJECT. To determine the ratio of the friction between two 
surfaces to the pressure holding them together. 

Apparatus. Rectangular wooden block ; board with uniform 
surface, with support for use as inclined plane ; spring balance 
(2000 g.) ; fish line ; block of weights ; meter stick. 

Introductory : 

Heavy loads on a wagon press down and increase the 
friction at the axles. The ratio between the friction and 
the pressure causing it, is called the coefficient of friction. 

This fraction has different values according to the 
kinds of surface in contact. For instance, there is more 
friction between rubber soles and a polished floor than 
between leather soles and the same floor. The man with 
the rubber soles can walk up a steeper plank, but even he 
will begin to slip when the pitch of the plank is increased 
to a certain definite angle. The leather soles slip at 
a smaller definite angle of pitch. 

The coefficient of friction may be found, either by 
measuring both friction and pressure directly, or by find- 
ing the angle of elevation of the surface of one body, at 
which the weight of a second body will just cause the 
latter to slip down the inclined surface of the first. 

Experimental : 

(a) A hard wood block, with various weights upon it, 
is dragged over the surface of a smooth horizontal board 
by means of a cord attached to the block and to a spring 
balance. If the block is kept moving at a uniform speed, 



114 



LABORATORY EXERCISES 



the reading of the balance will show the amount of the 
friction between the surfaces. The pressure between the 




Fig. 45. 

surfaces is the weight of the block plus the load placed 
upon it. Several weights ranging from 100 to 1000 grams 
should be used to load the block. From these readings 
the coefficient of friction may be found by dividing the 
friction by the pressure causing it. 

(5) Using the same block and board, with a support to 
adjust the board to any desired inclination, the board may 




Fig. 46. 

be raised gradually until the unloaded block will just 
slide down with uniform motion if the board is constantly 
tapped with the finger. This angle is called the limiting 



COEFFICIENT OF FRICTION 



115 




Fig. 47. 



angle of friction. Referring to Fig. 46, A and BO should 
be measured. 

When a body rests on an inclined plane, its weight, w, 
is resolved into two component forces. One of these, p, is 
perpendicular to the plane and 
produces pressure upon it. 
The other component / acts par- 
allel to the plane and toward the 
lower end. As this is the only 
component of the force that acts 
in the direction in which the' 
body on the plane may move, it 
is evident that only this force needs to be balanced to 
keep the body from moving down the plane. Therefore, 
at the limiting angle, the component / of the weight w, 
as it urges the block down the plane, just balances the 
friction. 

It will be readily seen (Fig. 47) that the triangles fwp f 

f BO f 

and ABO are similar. Hence, ^-= — — . But - is the 

p AO p 

friction divided by the pressure and is, therefore, the 

quantity we seek. Its value, then, may be calculated by 

dividing the height of the plane by the length of the base. 

Record the readings in tabular form near the top of the 

left-hand page. 



Observations 
Part (a) : 

1 2 

Total pressure (block and weight) g. 

Beading of balance . . . . g. 

Part (6) : 

Height of plane 

Length of base • . • . 



Etc. 



9- 



st- 



cm. 
cm. 



116 LABORATORY EXERCISES 

Make a clear outline drawing of your apparatus and 
briefly describe your work in both (a) and (5). 

Place the calculated results in tabular form at the top 
of the right-hand page. 



Part (a) : 



Calculated Results 

Coefficient of friction ( ^ ) 

\pressurej 



Etc. Average 



Part (6) : 



Coefficient of friction ( — ^ — J 
\ base J 



Discussion : 

Is the coefficient of friction dependent upon the load ? 
Show why the ratio of the height to the base of the in- 
clined plane at the limiting angle is equal to the coefficient 
of friction. 

Conclusion : 

The coefficient of friction between and is 

(name materials) 



EXPERIMENT 32 

Vibrations of a Tuning Fork 

OBJECT. To determine the frequency of a given tuning fork. 

Apparatus. A low frequency tuning fork (not over 128 V.P.S.) 
with considerable amplitude of vibration, preferably made of bell 
metal, and with a bristle or stylus attached ; oval piece of wood; 
glass plate smoked ; pendulum beating known fraction of a second, 
provided with a stylus ; rigid clamps for tuning fork and pendu- 



VIBRATIONS OF A TUNING FORK 117 

lum ; holder and track for glass plate ; candle, or cake of " Bon 
Ami." 

Note. — Apparatus dealers furnish sets of the above ap- 
paratus. 

Introductory : 

A knowledge of the number of vibrations correspond- 
ing to each musical note is essential to the understanding 
of the Physics of Sound. While the ear may be trained 
to estimate very closely the pitch of the tuning fork, the 
eye is not quick enough to count its vibrations. By pro- 
viding the fork with a tracing point and by drawing pre- 
pared glass or paper under the fork at right angles to the 
direction in which the fork is vibrating, each complete back 
and forth vibration of the fork will be represented by a 
wave-shaped mark. If a pendulum provided with a trac- 
ing point is so placed that it also vibrates across the glass, 
the distance the glass moved during the known period of 
the pendulum is also recorded. Then the number of vi- 
brations of the tuning fork in that period may be counted. 

Experimental : 

The best way of preparing the glass is to rub over it a 
thin coat of "Bon Ami" or of whiting and alcohol, and 
allow it to dry. The apparatus should then be carefully 
inspected and adjusted so that the tracing points of both 
the fork and the pendulum will sweep across the plate in 
as nearly the same line as they can without interfering 
with each other. The tracing points must bear on the 
surface hard enough to scratch away the coating, but not 
with pressure enough to check the motion of either fork 
or pendulum. This may be tested by setting each in 
vibration with the glass at rest. 

The fork is best set vibrating by placing between the 



118 LABORATORY EXERCISES 

prongs an oval stick of wood, thick enough to spread the 
prongs the desired amount, and then suddenly pulling it 
out. 

When all adjustments are made, set pendulum and 
tuning fork in vibration and with a steady, even motion 
draw the glass along the track at such a rate as to have 




Fig. 48. A Vibrograph. 



at least one complete swing of the pendulum, back and 
forth, recorded on the glass. Remove the glass, to permit 
others to use the apparatus. 

The number of complete wave forms traced by the fork 
between two successive points where the pendulum wave 
crosses the tuning fork wave, is the number of vibrations 
made by the tuning fork in the period of the pendulum. 

Place in tabular form, near the top of the left-hand page, 
the time of the pendulum period and the number of vibra- 
tions recorded each time during that period. 

Observations 
Trial \ 23 4 

Observed vibrations . 

Period of pendulum . 

Number of fork . . . 



VIBRATIONS OF A TUNING FORK 119 

Make a simple drawing of your apparatus and describe 
briefly the essentials of the method. 

Calculate the average number of vibrations for the 
period of the pendulum, and from the average find -the 
number of vibrations per second. Record the calculated 
results at the top of the right-hand page. 

Calculated Results 

Average number of vibrations in sec. was . . 

Frequency of fork (vibrations per second) . . . 



Discussion : 

(a) Explain fully why a complete wave trace of the 
fork stands for one vibration of the fork. 

(J) Why does a half wave trace stand for the period 
of the pendulum ? 

Conclusion : 

The frequencjr of fork No. is vibrations per 

second. 



120 LABORATORY EXERCISES 

EXPERIMENT 33 

The Velocity of Sound in Air 

OBJECT. To determine the approximate velocity of sound in the 
open air at the existing conditions. 

Apparatus. Pendulum (f sec.) with large-faced bob 1 and 
mounted in a shallow box ; pair of field glasses ; measuring tape ; 
two short pieces of board ; thermometer. 

Introductory : 

A flash of lightning is usually seen before the thunder, 
the sound accompanying the electric discharge, is heard. 
The steam escaping from the whistle on a distant loco- 
motive may be noticed several seconds before the sound 
reaches our ears. The flash of a gun is evident before 
the sound of the discharge is heard. All these illustra- 
tions show that sound travels much more slowly than 
light, and that an appreciable interval is required for a 
sound to travel any considerable distance. Since light 
has such great velocity, the time required for it to travel a 
part of a mile is not measurable by any ordinary means, 
while the comparatively slow-.travelmg sound takes a 
noticeable time for the same distance. These relative 
velocities make possible a simple method for determining 
the number of feet per second traveled by a sound. 

Experimental : 

Mount the pendulum beating three fourths of a second 
in a shallow wooden box with the cover removed. Stretch 

1 In case a pendulum with a brass bob is not available, a pendulum 
may be made with a 5-lb. slotted weight and a wooden bar, or a good bob 
could be cast of lead with a small brass curtain rod inserted, in the cover 
of a coffee tin or lard pail. Whatever large-faced bob is used, one face 
should be painted a blue similar to that used in the enameled street signs. 



THE VELOCITY OF SOUND IN AIR 121 

across the box opening an opaque white cloth and in it 
make a hole the shape and size of the pendulum bob at 
the center of its vibration. At the back of the hole and 
on the bottom of the box arrange a white background. 
The exposed face of the bob should be painted blue, since 
this color will be readily seen as the bob swings across 
the opening. 

Set the pendulum about 500 feet away, so placed that the 
bob of the pendulum is several feet from the ground. One 
student is stationed at the box with two short boards and 
strikes them together so as to produce a sharp sound every 
time the bob is at the center of its swing. 

Observers should move either toward or away from the 
pendulum until a position is obtained where the successive 
sounds produced coincide with the successive swings of 
the bob across the opening. This means that the sound 
produced at the center of one beat of the pendulum 
reaches the observer at the center of the next beat. Then 
during the time of one beat, the sound travels the distance 
of the pendulum from the observer. Field glasses will be 
necessary to see clearly the swing of the bob across the 
opening. 

Make one determination with the wind, and one against 
it, and record the distances as measured with a tape. 

Take the temperature of the air and record in the table 
of observations. 

Observations 

Distance of observer to pendulum, ivith wind . ft. 

Distance of observer to pendulum, against wind ft. 

Temperature of air ° (7. 

Make drawings showing how the pendulum was set up 
and describe the method of the experiment. 



122 LABORATORY EXERCISES 

Calculated Results 

Average distance traveled by sound in f second ft. 

Velocity of sound per second ft. 

Conclusion : 

The velocity of sound per second in the open air at °0. 

was 



EXPERIMENT 34 

Sympathetic Vibrations 

OBJECT. To set a tuning fork into vibration by sympathetic 
vibrations with another fork of the same frequency. 

Apparatus. Two tuning forks of the same frequency, 1 as 
256 V.P.S. ; tuning fork of different frequency, as 384 V.P.S. ; 
flat cork about 2" in diameter; 500-gram weight or iron ball 
with fish line for suspension ; support for hanging weight. 

Introductory : 

When the loud pedal of a piano is pressed, dampers are 
lifted from the strings so that the strings can vibrate 
freely. Then a note sung into the piano will make one 
wire vibrate in response, so that a note of the same pitch 
can be heard. The sound vibrations produced by the 
human voice have been the stimulus to the production 
of a sound by the vibration of one of the piano wires. 

1 Note to Instructor. — Two forks stamped with the same frequency- 
number will rarely vibrate at the same rate without filing notches in the 
end of one of them. Do this by taking two forks that sound nearly alike 
and than raise the pitch of the lower (flat) fork by filing the outer end 
of one prong. Then stamp or file an identifying number on the handle 
of both forks. Always give out together that pair of forks for this 
experiment. 



SYMPATHETIC VIBRATIONS 123 

Since the stimulating sound and the sound produced have 
the same pitch (frequency of vibration), this is a case of 
sympathetic vibrations. Tuning forks are very convenient 
instruments for studying sympathetic vibrations, for their 
rate of vibration per second is known. Usually the fre- 
quency number is stamped at the base of the two prongs. 

Experimental : 

(a) Suspend a 500-gram weight (or a ball of about the 
same weight) by a light, strong cord about a meter in 
length. 

When the weight is at rest, give it a light tap with a 
lead pencil, noting the direction in which the weight be- 
gins to move or vibrate. When the weight is at the 
center of its swing and moving from you, tap again. Con- 
tinue in this manner until the weight has received about 
twenty gentle taps. What is the effect upon the vibra- 
tions of the suspended weight ? From what source did 
the weight get its impulses ? 

With the weight again at rest, give it, without paying 
any attention to the intervals, twenty more gentle taps, 
hitting the weight just as it happens to be coming toward 
or going away from you. What is the effect on the vi- 
bration of the weight ? Compare the regularity in time 
of this second tapping with that of the first. What rela- 
tion existing between the regularity of the tapping and the 
vibration of the weight, caused such a marked effect in the 
first case ? 

(5) The following directions must be followed exactly 
in order to secure the desired result. Study them thor- 
oughly before beginning the experiment. Examine the 
forks to see that the same number is marked on the stem 
of each. 

(1) Hold the two forks by the stem, not allowing the 




124 LABORATORY EXERCISES 

fingers to touch any other part of the fork (in order to 
avoid heating). 

(2) Set the fork held in the right hand into vigorous 
vibration by striking the end of one of its prongs sharply 
against a cork on the desk. 

(3) Steady the fork in the left hand by allowing 
the hand to rest against the desk with the fork held 

horizontally. 

(4) Bring the vibrating 
fork into a position paral- 
lel to the other fork, with 
Fig. 49. the prongs extending in 

an opposite direction and 
the two forks about ^ of an inch apart (Fig. 49). 

(5) After the forks have been in this position while 
you count three, slowly bring the left-hand fork near 
the ear and determine whether it has been set into 
vibration. 

(6) If the first trial has not been successful, repeat the 
work. 

Apply to the instructor for a tuning fork of different 
frequency from that of the two forks used. With this 
fork and one of the former ones repeat the experiment, 
noting the success of your efforts. 

Make a drawing showing the forks in the position 
where sympathetic resonance was obtained. Write a full 
description of the experiment and its results. 

Conclusion: 

Answer the italicized question in Part (a). What must 
be true of the frequencies of two forks in order that one 
of them may be set into sympathetic vibration by the other? 



THE WAVE LENGTH OF A SOUND 125 

EXPERIMENT 35 

The Wave Length of a Sound 

OBJECT. To determine, at the temperature of the room, the 
length of a sound wave from a C tuning fork (256 V.P.S.). 

Apparatus. Glass resonating-tube about 12" long and 1" to 
1^" in diameter, with lower end closed by a 1-hole rubber stopper 
carrying a glass delivery tube with rubber connection and pinch- 
cock between the two sections ; beaker ; ring stand and clamp with 
jaws lined with cork; C tuning fork, 256 V.P.S. ; flat cork about 
2" in diameter. 

Introductory : 

When a prong of a tuning fork is vibrating, the prong 
makes a forward and a backward movement in completing 
one vibration. The vibrating prong sets the adjacent 
air vibrating longitudinally, and finally the sound wave 
reaches our ear. By using a glass tube with the lower 
end closed by water, we get a vibrating air column whose 
length can be readily measured. When the vibrating fork 
is held over the open end of the tube, the air column 
within is set into vibration. The sound wave starting 
from the prong travels down the tube to the water surface, 
where it is reflected and travels back again to the vibrat- 
ing prong. By varying the length of the air column, it is 
found that a column of certain length greatly intensifies or 
reenforces the sound of the tuning fork. This reinforce- 
ment or resonance is due to the reflected wave adding its 
sound to the sound being produced by the vibrating prong. 
To get the maximum intensification or resonance, the im- 
pulse started by the forward movement of the prong must 
travel down the tube and back again in time to reenforce 
the prong in its backward motion. Thus during the first 



126 



LABORATORY EXERCISES 



half vibration of the prong, the sound produced by it has 
traveled twice the length of the air column. During a 
whole vibration the sound would travel four times the length 
of the air column. The distance traveled hy a sound, 
while the body producing it is making one vibration, is 
the wave length of that sound. From this discussion it 
can be seen that the length of the vibrating air column is one 
quarter the wave length of the sound when the air column is 
adjusted to the point of maximum resonance. 



cO=CJ 



Experimental : 

Arrange the apparatus as in Fig. 50. To avoid disturb- 
ing sounds, the jaws of the clamp should be lined with 
cork and the delivery tube should 
always- be dipping into the water of 
the beaker. 

Start with the resonating tube 
nearly full of water, and, by letting 
the water out slowly through the de- 
livery tube, find a level which will 
give the strongest reenforcement of 
the sound emitted by the fork. In 
making this determination, set the 
fork vibrating loudly by striking it on 
the large cork, and hold the fork just 
over the top of the tube with the 
prongs parallel to the surface of the 
water. In case too much water runs 
out of the resonating tube, pour some 
^^ back from the beaker. In order to 
determine whether it is your air 
column or that of your neighbor 
which is sounding, keep moving your fork over the mouth 
of the tube and then away from the tube. 



c-i 



Fig. 50. 



THE WAVE LENGTH OF A SOUND 127 

When the precise level for the loudest resonance is 
found, measure in inches the length of the air column for 
this position and the internal diameter of the tube. 

Keeping the same fork, exchange the rest of your 
apparatus for that of another student and make a second 
determination of the level of loudest resonance. Measure 
as before and record. 

In the tabular form near the top of the left-hand page 
record the vibration number stamped on the fork and also 
the temperature of the room. 



Observations 

Length of resonant air column . 
Internal diameter of tube .... 
Frequency of fork (vibration number} 
Temperature of air of room .... 



it 



Make a drawing of your apparatus, showing the position 
of the fork and the length of the air column at the point 
of maximum resonance. Write a simple description of 
how the experiment was done. 

The measured length of the air column is not quite 
correct for the distance traveled by the sound in one 
quarter of a vibration. It actually travels a little farther, 
owing to the reflection from the sides of the tube and the 
spreading at the open end. Adding 0.4 of the diameter 
of the tube to the length makes the necessary correction. 



Calculated Results 

Correction for air column (0.4 diam.*) 
Corrected length of air column . 

(length + 0.4 diameter) 
Wave length produced by fork in air . 

(4 x corrected length) 



II 



128 LABORATORY EXERCISES 

Discussion : 

By reference to a lettered diagram showing a prong of the 
fork and an air column, tell why four times the length 
of the resonating column is taken as the wave length of 
the fork. 

Conclusion : 

The length of a sound wave from a C fork (256 V.P.S.) 

at ° C. w r as inches. 

(average) 

Velocity of Sound* — The wave length of sound pro- 
duced by a fork multiplied by the number of its vibra- 
tions per second (frequency) gives the velocity of sound 
per second. 

If the instructor so directs, make this calculation, using 
the data already obtained. Remember to divide the 
product by 12 in order to express the velocity of sound in 
feet per second at the temperature of the room. 



LAWS OF VIBRATING STRINGS 129 

EXPERIMENT 36 

Laws of Vibrating Strings 

OBJECT. To find how the frequency depends upon (a) the length 
when the tension is constant, (b) the tension when the length is 
constant. 

Apparatus. A simple sonometer, like the apparatus used in 
Experiment 7, page 37, modified to use with shorter wire and a 
meter stick; bridge for sonometer ; steel piano wire, about 26 B. 
& S. gauge ; slotted weights running to 5 kg. or about 10 lb. ; 
tuning forks 1 C (256 V.P.S,), A (426£ V.P.S.), and C (512 
V.P.S.) ; flat cork, 2". 

Introductory : 

No form of music is more appreciated than that pro- 
duced by stringed instruments. There is a fascination in 
watching the play of a violinist's fingers as he changes 
the lengths of the vibrating strings. In the preliminary 
tuning of the violin the strings are tightened by putting 
more pull or tension upon them. All these adjustments, 
made so readily by the practiced violinist, are in accord- 
ance with a few simple laws relating to vibrating strings. 
These laws may be determined in the laboratory by the 
use of simple apparatus and a little patient observation. 

Experimental : 

Arrange the apparatus as in Fig. 51, placing the bridge 
(i?) about 60 cm. from the fixed end (.A). Add enough 
weight to stretch the wire tight so that the wire will give 
a clear note when it is plucked. 

1 A G fork (frequency 384) would be recommended for this experiment 
in preference to the A fork, were it not for the relative cheapness of the 
latter. 



130 



LABORATORY EXERCISES 



(a) Law of Lengths* — Set the string AB vibrating 
by plucking it with your first finger. Strike a C tuning 
fork (frequency 256) on a flat cork, and note whether or 
not the fork and the wire give sounds of the same pitch. 
This unison can be told by the absence of beats. If the 
sounds are not in unison, increase the tension by adding 
more weight, and try again. Continue in this manner 
until unison is obtained, shifting the bridge a little, if 
necessarjr. Then the string and the fork are making the 



B 




Fig. 51. 



same number of vibrations per second. Measure and 
record the length AB of the string which gives 256 vi- 
brations per second. 

With the tension remaining the same, adjust the length 
of the vibrating string so that it is in unison with an A 
fork (frequency 426). Do this by moving the bridge. 
Measure and record the length of the vibrating wire. Is 
this wire which gives 426 vibrations shorter or longer than 
the wire with the frequency of 256 ? 

Again vary the length of the vibrating wire so as to 
bring it into unison with a C f fork (frequency 512). 
Measure and record. How does this length compare with 



LAWS OF VIBRATING STRINGS 131 

the length of a string which makes half as many vibra- 
tions per second, the tension remaining the same ? 

(5) Law of Tensions. — Record the weight which in 
Part (a) gave the tension on the wire with a frequency of 
512. Keeping the vibrating wire the same length, grad- 
ually decrease the tension by removing weight, until the 
wire is in unison with the C fork (frequency 256), as 
nearly as the weight permits. Record the tension. 

Find the ratio of the square roots of the two tensions. 
What else has the same ratio when the length of the vi- 
brating string is kept constant ? 

If there is time, and if the instructor so directs, verify 
the relation just formed by putting such a tension on the 
wire as will make it vibrate in unison with an A fork 
(approximate frequency 426). 

Observations 
Part (a): Law of lengths (tension constant). 



'ORK 


Frequency 


Length of "Wire in Unison 







em. 


A 




cm. 


C 




cm. 



Part (5): Law of tensions (length constant). 

Frequency of "Wire Tension (Weight) 

512 

256 
426 



Make a diagram of your apparatus. Describe briefly 
the method in each part of the experiment. 



132 LABORATORY EXERCISES 

Calculated Results 
Part (ft) : 

Frequency of Wire Square Root of Tension 

512 
256 
426 

Discussion : 

What is the quickest way of raising the pitch (fre- 
quency) of a violin string ? What is the effect of tight- 
ening a violin string ? Explain. 

Conclusion : 

State (a) the relation of pitch (frequency of vibration) 
to the length of a vibrating string when the tension is 
constant; (ft) the relation of pitch to the square root of 
the tension when the length is constant. 






MEASUREMENT OF CANDLE POWER 133 

EXPERIMENT 37 

Measurement of Candle Power 

OBJECT. To determine the candle power of a lamp by means of 
a Jolly or a Bunsen photometer. 

Apparatus. A jolly 1 or a Bunsen photometer; 2 incandes- 
cent lamps, one of known candle power. If electricity is not 
available, a standard candle and an oil lamp may be used. The 
ordinary paraffin candle, " 6's " or " I2's," are about 1.25 candle 
power. 

Introductory: 

The ordinary incandescent lamp is rated at 16 candle 
power. This means that it gives 16 times as much light 
as one standard candle. If the candle is placed on one 
side of a translucent screen and the lamp on the other, the 
screen can be moved to a position where it is equally illu- 
minated on both sides. The screen receives the same 
intensity of illumination from both lights, but the greater 
candle power of the lamp permits it to be much farther 
from the screen than the candle. If the latter is 20 cm. 
from the screen, then the distance of the lamp will be 
found to be 80 cm. It is interesting to note that the 

1 The Jolly photometer consists of two slices of paraffin about 5 cm. 
square, cut from blocks of "Parawax," and of equal thickness, separated 
by a sheet of tin foil (Fig. 52). These disks are mounted at the center of 
a block about 18 cm. long and held in place by a rectangular hood of 
tin (T) nailed to the block (W) (Fig. 53). The junction of the two 
blocks is viewed through an opening in the tin hood, at the center of the 
screen. The block and hood should be painted black. The different 
photometers in the laboratory should be so placed that each screen re- 
ceives light from its own lamps only. 

The authors are indebted to Mr. W. K. Pyle of the Morris High 
School, New York, for the simple method of mounting the Jolly screen 
given above. 



134 LABORATORY EXERCISES 

squares of these distances from the screen have the same 
ratio as the relative candle power of the two lights : 

Illuminating Power Illuminating Power Square op Candle Square of Lamp 
op Candle op Lamp Distance from Distance from 

Screen Screen 

1 : 16 :: 400 : 6400 

Hence the ratio of the illuminating poiver of two lights 
equals the ratio of the squares of their respective distances 
from the equally illuminated screen. 

The screen device for determining relative candle power 
is known as a photometer. Owing to the difficulty of 
getting candles of exact candle power, the candle power 
of lamps in commercial practice is usually found by com- 
parison with standard incandescent lamps, whose candle 
power has been accurately determined. 

Experimental : 

If the photometer differs from that shown in Fig. 52, the 
instructor will give directions for its use. In the Jolly 
photometer (Fig. 52), two slices of paraffin, separated by 



1 1 1 1 > 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ■ 1 1 1 1 1 1 1 1 1 1 1 1 1 nTT 






1 ' ' | ' ' ' ' I ' ' ' ■ I ' ' ' ' i ' i ■ ' j i ' ' j iT-r-r-r-p- 



Fig. 52. 

a sheet of tin foil, are used for the translucent screen. 
The tin foil prevents the light received from one lamp 

from illuminating the other side of the screen. 

The light is reflected instead, and intensifies the 



- 



p 

illumination in the half of the paraffin facing the 

\ ~ w lamp. When the two halves of the screen are 
equally illuminated, the two halves of the exposed 



"~ijr 

Fig. 53. ends will have the same shade 



MEASUREMENT OF CANDLE POWER 135 

Start with an incandescent lamp of known candle power, 
and compare with it a lamp of unknown candle power, as 
follows: 

(1) Place the incandescent lamp of known candle 
power in one socket and the unknown lamp in the 
other. Both sockets should be connected to the same 
current outlet. 

(2) Slide the screen along the meter stick, until -the 
two halves of the paraffin are of the same shade of bright- 
ness. Record in tabular form, near the top of the left- 
hand page, the position on the meter stick of the standard 
lamp, that of the unknown lamp, and that of the screen. 

(3) Move the lamp away from its position and make a 
second independent setting. The average of each pair of 
distances obtained should be used in making the final cal- 
culation. 

Observations 

Number of Position of Posit ton of Posit roN of Candle Power 

Beading Standard Unknown Screen of Standard 

1 

2 . 



Make a drawing, showing the essential parts of the pho- 
tometer, and describe how you used it. 

Using the ratio method given in the " Introductory," cal- 
culate the candle power of the unknown lamp, placing the 
calculated results at the top of the right-hand page. 

Calculated Results 

Average Distance Average Distance /Standard^ /Unknown\2 

of Standard of Unknown V Distance / \ Distance / 



Candle power determined for unknown _ 



136 LABORATORY EXERCISES 

Discussion : 

Demonstrate the relation between the distance and the 
intensity of illumination. 

How would you determine the candle power of a 
lantern ? 

Conclusion : 

The candle power of lamp No. is . 



EXPERIMENT 37 (Alternative) 

Measurement of Candle Power 

OBJECT. To determine the candle power of a lamp by means 
of the Rumford photometer. 

Apparatus. Ring stand ; vertical screen ; meter stick ; 
2 incandescent lamps, one of known candle power. If electricity 
is not available, a standard candle and an oil lamp may be used. 
The ordinary paraffine candle " 6's " or " 12's " are about 1.25 
candle power. A small lantern is a very desirable form of the oil 
lamp for this experiment on account of its candle power and 
its safety for laboratory use. 

Introductory: 

A 16 candle power lamp is one which gives 16 times as 
much light as one standard candle. If a candle and a 
lamp are both placed on the same side of a rod, they will 
each cast a shadow of the rod on a screen placed behind 
it. Each will then illuminate the shadow cast by the 
other. If the shadows are equally dark, then the screen 
receives the same intensity of illumination from both 
lights. The greater candle power of the incandescent 



MEASUREMENT OF CANDLE POWER 137 

lamp, however, permits it to be much farther from the 
screen than the candle. If the latter is 20 m. from the 
screen, then the distance of the lamp will be found to 
be 80 m. 

It is interesting to note that the square of these dis- 
tances from the screen have the same ratio as the rela- 
tive candle power of the two lights : 

Illuminating Illuminating Square of Candle Square of Lamp 

Power of Candle Power of Lamp Distance from Screen Distance from Screen 

1 : 16 :: 400 : 6400 

Hence the ratio of the illuminating power of the two lights 
equals the ratio of the squares of their respective distances 
from the equally illuminated screen. 

The photometer which determines candle power by 
means of shadows cast upon an opaque screen is the Rum- 
ford photometer. 

Experimental : 

1. Place the upright rod of the ring stand about 10 cm. 
from the vertical screen. 

2. Place the lamp whose candle power is to be deter- 
mined at a distance of about 120 cm. from the screen. 

3. Place the standard lamp (or candle) in such a posi- 
tion that the two shadows of the rod formed on the screen 
shall be of the same intensity. These shadows should be 
near each other, but should not overlap. 

Measure with the meter stick the distance of the stand- 
ard lamp to the nearer of the two shadows, and the dis- 
tance from the unknown lamp to the other shadow. 
Record the results in tabular form near the top of the 
left-hand page. Why was the distance measured in each 
case to the nearer of the two shadows ? 



138 



LABORATORY EXERCISES 



4. Repeat the above test with the unknown lamp (or 
candle) successively at 100 cm. and 80 cm. from the 
screen. 

In case two incandescent lamps are compared, their 
sockets should be connected to the same current outlet. 

Observations 



Number 


OF 


Distance 


of Unkno 


WN 


Distance 


of Standard 


Trial 




from 


Screen 




from 


Screen 




1 








cm. 






cm. 


2 








cm. 






cm. 


3 








cm. 






cm. 



Candle power of standard . 

Make a drawing showing the essential parts of the 
photometer, and describe how you used it. 

Using the ratio method given in the " Introductory," 
calculate the candle power of the unknown lamp, placing 
the calculated results in tabular form at the top of the 
right-hand page. 

Calculated Results 



Number of 
Trial 



1 

2 
3 

Average 



/Standard \ 2 
V Distance / 



/ Unknown \ 2 
\ Distance / 



Candle Power 
of Unknown 



Discussion : 

Demonstrate the relation between the distance and the 
intensity of illumination. 



LAW OF REFLECTION OF LIGHT 139 



Conclusion : 

The candle power of is 

(name lamp) 



EXPERIMENT 38 

Law of Reflection of Light 

OBJECT. To determine the relation between the angle of in- 
cidence and the angle of reflection. 

Apparatus. Glass or metal mirror ; clamp or block for 
holding mirror ; sheet of clear glass the same size as the mirror ; 
pins ; ruler ; protractor. 

Introductory : 

When sunlight falls upon a mirror, the light is reflected 
in a definite direction. The relation between the direction 
of the light before and after it strikes the mirror is stated 
in the law of reflection. We can locate a particular re- 
flected ray by sighting along a ruler at the image of the 
object that is reflected. A line drawn along the edge of 
the ruler will mark the direction of the reflected ray. 

Experimental : 

(1) Draw a line across the middle of the right-hand 
page of your note-book. Mark it MM. Place the mirror 
perpendicular to the page with its reflecting surface on 
this line. Stick a pin upright in the page in front of the 
mirror and mark its position P. 

(2) Placing the head on the level of the page and 
opposite one of the lower corners of the book, sight along 
a ruler placed on the page at the image of the pin, as seen 
in the mirror. When the edge of the ruler is exactly in 



140 



LABORATORY EXERCISES 



a line with the image of the pin, draw a line along the 
edge of the ruler. 

(3) Repeat the operations described in (2), with the 
eye near the other lower corner of the book. 




Fig. 54. 



(4) Remove the mirror and substitute a transparent 
sheet of glass, carefully placing its front edge on the line 
MM. Protect the page behind the mirror from the direct 
light of the windows. Looking through this transparent 
mirror, insert a pin to coincide with the image of P. 
Mark the position of this pin P r . Remove the mirror 
and pins. 

(5) Continue each of the lines drawn along the edge of 
the ruler as solid lines to the mirror line MM, and con- 
tinue them as dotted lines behind the mirror until they 
meet. Do these solid lines represent incident or reflected 
rays ? 

From P draw a line to the intersection of each of the 
lines just drawn with the mirror line. Do these lines from 



LAW OF REFLECTION OF LIGHT 141 

P mark incident or reflected rays ? Connect P and P ! 
with a line, solid from P to the mirror and dotted from 
the mirror to P' . Mark the direction in which light is 
passing along each of the solid lines by an arrow on that line. 

(6) At the intersection of one of the solid lines with 
the mirror line, erect a perpendicular to the mirror line. 
The angle between the line coming from the pin to the 
mirror and this perpendicular line is called the angle of 
incidence. The angle between the reflected ray and this 
perpendicular is called the angle of reflection. Measure 
these angles with a protractor. 

Record in tabular form near the top of the left-hand 
page the measurements called for. 

Observations 

Angle of incidence 

Angle of reflection 

Distance of pin from mirror ...... cm. 

Distance of image from mirror cm. 

Angle between MM and PP f 

A simple sketch of the apparatus as seen from above 
should be made and the experimental operations described 
briefly. 

Discussion : 

Answer the italicized questions occurring in the ex- 
perimental directions. As seen in the mirror, from what 
point do the rays sighted along the ruler appear to come ? 
From what point do they actually come ? 

Compare the distance of the image of the pin as seen 
in the mirror with the distance of the pin itself. 

Conclusion : 

State the relation between the angle of incidence and 
the angle of reflection. 



142 LABORATORY EXERCISES 

EXPERIMENT 39 

Images in a Plane Mirror 

OBJECT. To compare an object with its image in a plane mirror 
with respect to size, distance, and form. 

Apparatus. Glass plate for mirror ; wooden block with slot 
for holding mirror in vertical position ; half-moter stick, or foot 
ruler ; pins. 

Introductory : 

The plane mirrors which hang on our walls give rise to 
some interesting questions. Why does your image in 
the mirror seem to approach you as you walk toward the 
mirror ? Why do you sometimes move your hand in the 
wrong direction when attempting to adjust something on 
your head ? Why is a long mirror desirable when you 
want to see your apparel from head to foot ? An answer 
to these questions will be found in* the study of the 
relations of the object to the image in a plane mirror. 

The image of an object is composed of the images of the 
points in that object. We can locate the image of each 
point in a transparent mirror by direct observation, as 
was shown in the experiment on the law of reflection. 

Experimental *. 

Draw a line across the middle of the right-hand page of 
your note-book. About two inches below this line of 
reference make a drawing of a quadrilateral set obliquely 
to the line, no side of the drawing to be less than 1|- inches 
in length. Number the corners 1, 2, 3, and 4, as in 
Fig. 55. 

Place the front of the glass plate along the line of 
reference. Place a pin at point 1 of your drawing and 



~3 



IMAGES IN A PLANE MIRROR 143 

set a second pin at the image of 1 as seen in the mirror. 
Mark the position of the image pin with a pencil dot and 
the figure 1'. Locate and mark the image of each corner 
in the same way. Connect these 
points by lines representing the 
images of the corresponding 
edges of the block. 

Shading the part of the note- 
book behind the mirror helps 
to secure a clear image in the Fi g- 55 « 

mirror. 

Compare the object and the image by means of the 
following measurements, which should be recorded in 
tabular form near the top of the left-hand page. 




Observations 



Lines 3-4 Lines 1-3 



Length of lines in object ..... 

Length of lines in image . , . . . 

Points 1 2 3 4 

Distance of object's points 
from mirror . . . . 

Points V 2' 3' 4' 

Distance of image s points 
from mirror . . . . 

Write a brief description of the method of the experi- 
ment on the left-hand page. 

Conclusion: 

On the second right-hand page, answer the following 
questions, using a complete sentence for each answer. 

1. What relation exists between the object distance 
and the image distance of a point from the mirror line ? 

2. Compare the size of the object and its image. 



144 LABORATORY EXERCISES 

3. Is the image formed by a vertical mirror erect or 
inverted ? (Before answering, consider your own image 
in a mirror.) Is the image of the pin in front of the 
mirror or behind it ? 

4. In which direction do the hands of a watch appear to 
turn when viewed in a mirror ? 

5. Are an object and its image in a plane mirror similar 
or symmetrical ? 

EXPERIMENT 40 

Reflection in a Concave Mirror 

OBJECT. To study the form and location of the images formed 
by a concave mirror. 

Apparatus. Concave spherical mirror of glass or metal, sup- 
ported in a vertical position ; two meter sticks so placed as to 
form a V with its apex beneath the center of the mirror ; two 
screens mounted so as to slide on the meter sticks — one screen 
is opaque and the other has cut in it a round or square window, 
over which is pasted very thin paper, with ink lines ruled across 
it at right angles and an ink mark in one of the four spaces formed 
by the intersecting lines ; candle or incandescent lamp, to be 
placed behind the translucent window. 

Introductory : 

The beam of light from a headlight, where the burner is 
quite near the surface of a concave reflector, is shaped like 
a cone with its apex behind the reflector. The beam from 
a searchlight may take a conical shape like that from the re- 
flector. It may, however, be made parallel, or it may even 
be brought to a brilliant focus at some point in front of the 
searchlight. These changes in the shape of the beam are 
possible because the distance between the light and its 



REFLECTION IN A CONCAVE MIRROR 



145 



concave reflector can be varied. The position of the light 
when the reflected rays are parallel is called the principal 
focus and the perpendicular distance from the principal 
focus to the mirror is the focal length of the mirror. For 
every case of reflection from a concave mirror a definite 
relation exists between the distance of the object, the dis- 
tance of its reflected image, and the focal length of the 
mirror. 

Experimental : 

(a) In a darkened room, the translucent screen, lighted 
from behind, is placed on one of the meter sticks at a con- 
siderable distance from the mirror. The opaque screen, 
on the other meter stick, is then moved backward and for- 
ward until a position is found where the most distinct image 




Fig. 56. 

of the lighted window is formed. Record the distance of 
each screen from the mirror, also whether the image is 
larger or smaller than the object and whether the image is 
erect or inverted. Note in this and in each of the follow- 
ing cases whether there is any image back of the mirror, 
as in the case of the plane mirror. 

(6) Find another pair of positions for the screens where 
the image will now be larger than the object, if it was 
smaller before, or vice versa. The same items are to be 
recorded as before. 



146 



LABORATORY EXERCISES 



(V) Another pair of positions is to be found where the 
image will be as nearly as possible the same size as the 
object, and a similar record made. 

(cT) The lighted window is next moved toward the 
mirror, until there is no image formed on the opaque 
screen at any distance, but an image appears to be formed 
behind the mirror. The position of the lighted screen 
and the general location and character of the image are to 
be recorded. 

(e) Finally the illuminated screen is removed, and the 
meter stick on which this screen rests is pointed through 
the window at some distant object outside. If the weather 
permits, the window should be open. The location of the 
image should be recorded. This image is at the principal 
focus of the mirror. 

All observations should be recorded in a table near the 
top of the left-hand page. Where distances are not meas- 
ured, record general position of object or image. 

Observations 





Trial 


Object Distance 


[mag-e 


Distance 


i mack erect or 
Inverted 


Image Enlarged 
or Diminished 


a 


cm. 




cm. 






b 


cm. 




cm. 






c 


cm. 




cm. 






d 


cm. 




cm. 






e 


cm. 




cm. 







Make a simple outline drawing of your apparatus. A 
view from above, showing the location of the mirror, 
screens, and meter sticks, will be sufficient. Describe 
briefly your observations, stating particularly anything 



REFLECTION IN A CONCAVE MIRROR 



147 



you observe about the images which is not recorded in the 
table above. 

For cases (a), (5), and (c), calculate, as decimals, the re- 
ciprocals of the object distance and of the image distance. 

From (e) calculate the reciprocal of the principal focal 
length. This is to be compared in each case with the 
sum of the other two reciprocals. All results are to be 
recorded in tabular form at the top of the right-hand page. 

Calculated Results 





Trial 


i l 


1 1 1 


1 


Object Dist. 


Image Dist. 


Object Dist. Image Dist. 


Focal Length 


a 










b 










C 











Discussion : 

(1) When an object is at the center of curvature of a 
concave mirror (that is, at the center of the sphere from 
which the mirror is cut), the image is at the same place 
and of the same size as the object. Do any of your read- 
ings give you the radius of the curvature of your mirror? 
If so, which trial ? 

(2) When the object is at a distance greater than the 
radius of curvature, describe the image as to whether it is 
real or virtual, erect or inverted, enlarged or diminished. 
State the location of the image with reference to the center 
of curvature and to the principal focus. 

(3) " When the object is between the center of curvature 
and the principal focus, the image is ... ' (Complete 
the statement, touching on each of the points noted in (2).) 

(4) " When the object is between the principal focus 



148 LABORATORY EXERCISES 

and the mirror, the image is ... ' (Complete as in (2) 
and (3).) 

(5) What kind of rays are reflected to the principal 
focus? Where must an object be to send rays of approxi- 
mately this character ? 

Conclusion : 

What is the relation between the reciprocal of the focal 
length of a concave mirror and the sum of the reciprocals 
of the object distance and the image distance? Give your 
answer both in words and in an algebraic form. 



EXPERIMENT 41 

Reflection in a Convex Mirror 

OBJECT. To study the form and location of the images formed 
by a convex mirror. 

Apparatus. Convex spherical mirror, mounted in a vertical 
position ; candle or incandescent lamp ; meter stick, with sliding 
opaque screen mounted on it. 

Introductory : 

A polished door knob reflects a distorted image of the 
objects in the room. Other bulging curved surfaces re- 
flect in a similar manner. Convex spherical mirrors are 
frequently used as pocket mirrors. 

Experimental : 

Place the candle or lamp in a considerable number of 
positions, at different distances from the mirror. At each 
position, observe the character and location of the image 



REFLECTION THROUGH A GLASS PLATE 149 

formed. As the object approaches the mirror, notice 
whether the image approaches or recedes. 

Make a simple sketch of the apparatus and give a brief 
description of your work; a tabulation of observations and 
results is not necessary, as these are to be summed up in 
the Conclusion. 

Conclusion : 

Make a general statement as to the character (real or 
virtual), position (erect or inverted), shape (similar to 
object or symmetrical with it), and location of the images 
formed by a convex mirror, 



EXPERIMENT 42 

Refraction through a Glass Plate 1 

OBJECT. To study the refraction of a ray of light through a 
thick, rectangular glass plate. 

Apparatus. Thick rectangular glass plate ; ruler ; pins. 

Introductory : 

When we look through a thick plate of glass, objects 
seem displaced to one side or the other. This is the effect 
of refraction. We may study this effect by marking the 
path of an oblique ray with two pins before it enters the 
plate, and then sight along a ruler to determine the emer- 
gent ray. 

1 Note to Instructor. If a qualitative treatment of refraction is desired, 
students should perform Experiment 42 or Experiment 43, or both. The 
quantitative treatment, as well as the qualitative, is provided for in Ex- 
periment 44. 



150 



LABORATORY EXERCISES 



Experimental : 

Place a rectangular plate of glass near the center of the 
right-hand page of your note-book, having two clear edges 
parallel to the top and bottom of the book. With a 
sharp, hard pencil, trace the outline of the plate of glass 
on the page. 

Near a corner of the upper edge, draw a line at an angle 
to the upper edge of the glass. On this line place two 

pins, several centimeters 
apart. Place the eye on a 
level with the glass plate. 
Looking through the glass, 
place a ruler between the 
block and your eye, so 
that you can sight along 
its edge at the other two 
pins, as seen through the 
glass. Trace the position 
of the edge of the ruler. 
Remove the glass and 




Fig. 57. 



pins. Continue the lines 
you have drawn until they 
have met the lines indicating the surfaces of the plate. 
Draw a line representing the path of the light through 
the glass, indicating by arrowheads on all lines, the 
direction in which the light is proceeding. 

On the diagram, at the point where the ray of light 
entered the glass, draw a dotted line perpendicular to the 
surface of the plate, and continue it part way across the 
rectangle representing the plate. 

At the point where the light ray emerged, draw a similar 
dotted perpendicular and continue it upward into the 
rectangle. These perpendiculars, drawn where the light 



REFLECTION THROUGH A PRISM 151 

ray enters or emerges from the plate, are known as 
normals. 

Write a brief description of the method of the experi- 
ment, referring to the diagram. No other drawing is 
necessary. 

Conclusion : 

Make a general statement regarding the relative di- 
rection of the entering and emerging rays, when the faces 
at which the light enters and emerges from the medium 
are parallel. 

A ray of light on passing from a rarer to a denser 

medium is bent the normal ; in passing from a denser 

to a rarer medium, the ray is bent the normal. 



EXPERIMENT 43 

Refraction through a Prism 

OBJECT. To study the refraction of a ray of light passing 
through a triangular glass prism. 

Apparatus. Triangular glass prism, about 7 cm. on a side 
and 1 cm. thick: ruler; pins. 

Introductory : 

Glass prisms at one time were hung as ornaments in 
front of windows. Objects outside, when viewed through 
a prism, seem displaced to one side or the other. This is 
the effect of refraction. We may study the effect of 
refraction by marking the path of an oblique ray with 
two pins before it enters the prism, and then, sighting 
along a ruler, determine the emergent ray. 



152 LABORATORY EXERCISES 



Experimental : 



Place a triangular glass prism near the center of the 
right-hand page of your note-book, having one edge of the 
prism parallel to the bottom of the page. Trace the out- 
line of the prism on the page with a sharp, hard lead pencil. 

A little to the right of the center of the left edge of the 
prism draw a line at an angle to the edge. Do not make 
the angle between the ray and the edge more than 45°, or 
total reflection may occur. On the line just drawn, place 
two pins several centimeters apart. 

Place the eye on a level with the glass prism. Looking 
through the glass, place a ruler between the prism and 
your eye, so that you can sight along its edge at the two 
pins as seen through the right side of the glass. The 
ruler should be moved until the two pins, as seen through 
the glass, appear in the same straight line. Trace the 
position of the edge of the ruler on the page. 

Remove the glass and the pins. Continue the lines you 
have drawn to the lines representing the right-hand edge 
and the left-hand edge of the prism respectively. Draw 
a line representing the path of the light through the glass, 
indicating by arrow-heads on all lines the direction in 
which the light is proceeding. 

On the diagram, at the point where the light entered 
the glass, erect a dotted line perpendicular to the surface 
of the glass, and continue it part way across the triangle 
representing the prism. 

At the point where the light emerged, draw a similar 
dotted perpendicular, and continue it into the triangle. 

These perpendiculars drawn where the light ray enters 
or emerges from the prism are called normals. Note the 
direction of bending of the light with reference to the 
normals at each surface of the glass. 



INDEX OF REFRACTION 153 

A brief description of the' method of tracing the ray 
through the glass should be written, but no drawing other 
than the diagram is necessary. 

Discussion : 

How is a ray of light bent with regard to the normal : 
(a) on entering a denser medium ? (5) on emerging into 
a rarer medium ? 

Conclusion : 

Is light bent by a triangular prism toward the apex 
(refracting angle) or toward the base of the prism ? 



EXPERIMENT 44 

Index of Refraction 
OBJECT. To determine the index of refraction of glass. 

Apparatus. Thick rectangular glass plate, or triangular glass 
prism (1 cm. thick) or both ; pins ; metric ruler. 

Introductory : 

When viewed through a thick plate of glass, objects 
seem displaced to one side or the other. This is the effect 
of refraction. This effect may be studied by marking 
the path of an oblique ray with two pins before it enters 
the plate, and then sighting along a ruler to determine the 
emergent ray. 

Light travels faster in air than in a denser medium like 
glass. The ratio of the velocity of light in air to the 
velocity of light in glass is termed the index of refraction 
of air to glass. This ratio is mathematically equal to the 
ratio of the sine of the angle of incidence (air to glass) 



154 



LABORATORY EXERCISES 



to the sine of the angle of refraction. In this experi- 
ment you will learn what is meant by the angle of 
incidence and the angle of refraction. You will con- 
struct and measure the sine of each of these angles. 
You can then calculate the index of refraction of glass, 
relative to air. 

Experimental : 

If a rectangular glass plate is used, follow the experi- 
mental directions given in Experiment 42, page 150. 
For a triangular prism, follow Experiment 43, page 152. 
Then complete the experiment according to the directions 
which follow. 

At the point where the light ray enters and the point 
where it emerges from the glass, perpendiculars to the glass 
surface (normals) have been erected. Indicate the angles 
between the incident rays and the 
normals as angles of incidence; those 
between the refracted rays and the 
normals as angles of refraction. 
Taking each intersection of a normal 
with the surface of the glass as a cen- 
ter, describe circles of as large radius 
as possible, without the circles inter- 
secting. From the intersection of 
each ray with its circle, drop a per- 
pendicular to the normal in that 
circle. This perpendicular is known 
as the sine of the angle of incidence 
or of the angle of refraction, as the 
case may be. 

With a metric scale determine the lengths of these sines 
and record near the top of the left-hand page in a tabular 
form like the following : 




Fig. 58. 



TOTAL REFLECTION 155 

Observations 

Sine of first angle of incidence cm. 

Sine of first angle of refraction cm. 

Sine of second angle of incidence .... cm. 

Sine of second angle of refraction . . . . cm. 

Give a brief account of the geometrical construction 
you have made. No further drawing is necessary. 

Calculate the index of refraction from air to glass, mak- 
ing use of the measurements made on each side of the 
glass ; in each case the index is the ratio between the 
sine of the air angle and the sine of the glass angle. 
Tabulate the results. 

Calculated Results 

First Case Second Case Average 

Index of Refraction 
Conclusion : 

The index of refraction of glass, relative to air is 

EXPERIMENT 45 

Total Reflection 

OBJECT. To observe total reflection and determine the critical 
angle for glass. 

Apparatus. Flat triangular glass prism, about 7 cm. on a 
side and 1 cm. thick, having a fine line drawn across the center 
of one of the narrow faces, at right angles to the broad faces ; 4 
pins ; ruler ; protractor. 

Introductory : 

The surface of a glass of water, viewed obliquely from 
below through the water itself, becomes bright like a sil- 



156 



LABORATORY EXERCISES 



vered mirror, and reflects like one, when a certain position 
has been reached. Ice, so transparent when in a block, 
is white when powdered. Both of these appearances re- 
sult from total reflection. Light passing through a dense 
medium, as water, ice, or glass, to the surface of a medium 
less dense, as air, is refracted away from the perpendicular. 
That is, the angle of refraction, under these circumstances, 
is always greater than the angle of incidence. When the 
angle of incidence reaches a certain value, the refracted 
ray will lie along the surface. This value of the angle 
of incidence is called the critical angle. If the angle of 
incidence increases to a value greater than the critical 
angle, the light is totally reflected, instead of being re- 
fracted. By finding experimentally the least angle of 
incidence at which total reflection takes place, the critical 
angle can be found, though the value obtained from this 
experiment is an approximate one only. 

Experimental : 

(a) A flat triangular prism of glass is placed on the 

center of the right-hand page of the note-book. The 

directions which follow must be fully understood before 

a c tlie experiment is begun 

and must be exactly fol- 
lowed to secure accurate 
results. 

Close to the prism on 
the side AB insert a ver- 
tical pin Qt?) firmly in 
the paper. Near the 
center of the side AC 




Fig. 59. 



a vertical line (0) is ruled on the glass. Placing the eye 
on the level of the book, move the head until, looking 
through the side BC, an image of the pin (5?) is seen 



TOTAL REFLECTION 157 

reflected in AC. Note the appearance of AC when the 
head is in this position. The observed image is the result 
of the total reflection of light passing through the glass 
from (jt>) to the surface AC 

(£) Move the head sideways until the reflected image 
of the pin suddenly disappears. Continue moving the 
head in the same direction. Does the irgage again ap- 
pear ? Move the head in the opposite direction. Does 
the image now appear ? Beyond the point where the 
image suddenly disappeared, the light rays from the pin 
were refracted in the ordinary wa} r , and the pin might 
have been seen by looking through the side AC The 
particular angle of incidence on the surface AC of rays 
from the pin at which total reflection begins and refrac- 
tion ends, is called the critical angle. It is now to be 
determined. 

(<?) Keeping the side AB always closely against the 
pin, move the prism and the head into various positions, 
until the reflected image is just about to coincide with 
the vertical mark on A C as the image disappears. When 
you are sure that you have located this position correctly, 
insert two pins (j/) and Qp") so they are in a straight 
line with the mark on- AC, as seen through the glass. 
These pins, then, lie in the line taken by the reflected 
ray after it leaves the glass. 

Holding the prism firmly to the paper with the left 
hand, trace its outline and mark on AC the exact loca- 
tion of the vertical mark (0) on that face. Removing 
the prism, draw a line from (j?) to the marked point (<?), 
representing the path of the ray incident at (0) from (jt?). 
Draw a line through the pins (jt/) and (jp") to BC, and 
from the point of intersection with BC draw a line to (0). 
Place an arrow head on each line to show the direction 
of the light in each case. 



158 LABORATORY EXERCISES 

At (o) erect a perpendicular to AO. With a protractor 
measure the angle of incidence (which is the critical angle 
if your work has been done correctly) and the angle of 
reflection in the glass. Record the readings of the pro- 
tractor on the figure. 

If time permits, repeat the process of finding the critical 
angle, using the next page of the note-book. 

No table of observations is necessary, as all observed 
results are recorded on the drawing. Write a brief but 
complete description of your work, referring to the draw- 
ing, and mention any conditions that were observed which 
are not shown by the drawing. No sketch of the apparatus 
is necessary. 

Discussion : 

Through what kind of a medium must light pass in 
order to be totally reflected at the transparent surface of 
that medium ? 

Under these circumstances, if the angle of incidence be 
greater than the critical angle, what happens to the light ? 
If the angle of incidence is less than the critical angle, 
w r hat happens ? 

In total reflection, how does the angle of incidence 
compare with the angle of reflection ? 

Conclusion : 

The critical angle of glass is °. 



STUDY OF A CONVERGING LENS 159 

EXPERIMENT 46 A 

Study of a Converging Lens 1 

OBJECT. To locate the principal focus of a converging lens and 
to study the images formed by such a lens, when the lens is at 
different distances from the object. 

Apparatus. Double convex lens, 10 to 15 cm. focus ; opaque 
screen ; half-meter stick ; screen with translucent window (see 
description under " Apparatus," page 166) ; meter stick, mounted 
as shown in Fig. 61 ; lens and screen holders to slide along the 
meter stick ; incandescent lamp or other light ; strip of paper 
more than twice the focal length of the lens. 

Introductory : 

Converging lenses are among the most useful parts of 
optical instruments, such as cameras, telescopes, and pro- 
jection lanterns, The first experience of most boj^s with 
a converging lens is the handling of a "burning glass." 
The parallel rays from the far distant sun enter the lens, 
and are so bent in direction that they converge to a point. 
This point of convergence of parallel rays is the principal 
focus of the lens. The focal length of a lens is the dis- 
tance from the lens to the principal focus. 

When we look through a converging lens at an object, 
we see an image of the object. The relations of the ob- 
ject and image vary according to the position of the object 
with reference to the principal focus. The relations are 
not hard to find and are interesting, because they explain 
the use of the converging lens in some of its important 
practical applications. 

1 This experiment is essentially qualitative in its character. Experi- 
ments 46 B and 47 provide for a quantitative treatment of the convex 
lens. Either one kind of .work or the other should be selected, as the 
performance of all three experiments would involve unnecessary repetition. 




160 LABORATORY EXERCISES 

Experimental ' 

(I) The Principal Focus. — If we assume that the rays 
from a fairly distant object are practically parallel, and 
that these rays on entering the lens converge to the prin- 
cipal focus, the location of a sharp image of the distant 
object on a screen tells us the position of the principal 
focus. Accordingly, set the lens on one of the main divi- 
sions of a half-meter stick, and move the screen until the 
most distant bright object which can be seen through the 
window is sharply focused on the screen. 
Note the distance between the lens and the 
screen (principal focus). Record this focal 
length in the table of observations near the 
top of the left-hand page. Take two more 
readings, moving the lens and screen each 

time. Record these readings, and the aver- 
Fig. 60. . . 

age of the three, which will be considered 

the focal length. A simple and very convenient form of 

lens holder is shown in Fig. (30. 

(II.) Relations of Object and Image* — On a strip of 
paper draw a line just twice the focal length of the lens in 
length ; in the middle of the line place a mark, the dis- 
tance of which from either end will be equal to the focal 
length. All distances in the remaining portion of the 
experiment are to be measured in terms of the focal length 
of the lens, by means of this marked line, and not by 
means of the numbers on the meter stick. 

At one end of the meter stick place an incandescent 
lamp or other light, and directly in front of the light a 
screen with a translucent window in it to serve as an 
object (Fig, 61). 

(a) Set the lens at its focal length from the illuminated 
screen. The object is now at the principal focus of the 



STUDY OF A CONVERGING LENS 



101 



lens. Move the opaque screen on the other side of the 
lens, and note whether or not an image is formed on this 
screen. The formation of an image means that the rays 
of light leaving the lens converge. If an image is not 
formed, the rays leaving the lens are either parallel or 
divergent. When the object is at the principal focus, what 
is the direction of the rays leaving the lens? (Recall the 
method of finding the principal focus.) 

(b) Move the lens nearer "the illuminated screen than 
in (a). The object is now within the principal focus. 
Move the screen to ascertain whether or not an image is 




Fig. 61. 

formed. Look through the lens at the illuminated screen 
and describe its appearance. In this case ivhat do you 
think is the direction of the rays leaving the lens? Explain, 
(e) Place the lens so that the object is at a distance of 
twice the focal length. Place the screen at an equal dis- 
tance on the other side of the lens. Is the image on the 
screen erect or inverted ? l Is it larger or smaller than the 
object ? When the object is at txcice the focal length from the 
lens com jj are (1) the relative distances from the lens of object 
and image. (2) the relative size of object and image. At 



1 In case a sharp image is not formed at twice the focal length, find the 
shortest distance between the object and the screen at which a distinct 
image of the object can be formed on the screen. Compare the object 
and ima^e distances with each other and with twice the focal length. 



162 LABORATORY EXERCISES 

what distance from a camera lens would you place a drawing 
in order to obtain a photographic copy of the same size ? 

(d) Move the lens in a little toward the object, so that 
it is at a distance from the object greater than the focal 
length, but less than twice the focal length. Move the 
opaque screen until a sharp image of the illuminated 
screen is obtained on it. Alongside the line already 
drawn on your strip of paper, lay off another line whose 
length is the distance between the lens and the image in 
this case. On this line also mark the object distance. 

Compare the image distance with twice the focal length. 
Note the relative sizes of object and image. When an 
object is at a distance from a lens greater than the focal 
length, and less than twice the focal length, (1) state the gen- 
eral location of the image in terms of the focal length, (2) 
compare the image and object as to size. 

(0) Move the lens to a point whose distance from the 
object is equal to the image distance obtained in (d). The 
object distance is now greater than twice the focal length. 
Slide the opaque screen into a position where a sharp 
image is formed. Note the relative sizes of object and 
image. Beside the line drawn in (d), lay off another line 
on which the object and image distance in this case are 
marked. Compare the image distance in this case with 
twice the focal length and with the focal length. When 
an object is at a distance from a lens greater than twice the 
focal length, (1) state the general location of the image, (2) 
compare the object and image as to size. Conjugate foci of 
a lens are points so located in reference to the lens that, 
if the object is placed at either point, the image will be 
located at the other. State two cases of conjugate foci 
shown in this experiment. 

In a table near the top of the left-hand page, the read- 
ings of focal length are to be entered. Immediately 



STUDY OF A CONVERGING LENS 163 

beneath this the strip of paper on which the various dis- 
tances have been recorded, is to be pasted by one end. 
Each line on the strip should be marked to indicate just 
what distances it records. 

Observations 

12 3 Average 

Focal length of lens cm. cm. cm. cm. 

A brief description of the work done in each part of the 
experiment should folio w the " Obser vations. " Any obser- 
vations not recorded on the strip or in the table should be 
included in the description. The description should be 
accompanied by a drawing showing the apparatus when 
the principal focus was being determined, and a drawing 
showing the location of lens, screens, and lamp in position 
for one of the cases where an image was formed. 

Discussion : 

Answer under this heading all questions in italics con- 
tained in the experimental directions. 

Where will the screen for a stereopticon be located with 
reference to the focal length of the objective lens ? Where 
will the lantern slide be located ? 

Conclusion : 

What is the least distance from a converging lens at 
which an object can be placed in order that a real image 
may be formed ? 

State a general relation between the sizes of the object 
and image, and their respective distances from the lens. 



164 



LABORATORY EXERCISES 



EXPERIMENT 46 B 

Focal Length of a Converging Lens 

OBJECT. To locate the principal focus and determine the focal 
length of a converging lens. 

Apparatus. Double convex lens, 10 to 15 cm. focus; lens 
holder ; screen ; screen holder; half-meter stick; the lens and 
screen holders should fit and slide along the half-meter stick. 

Introductory : 

A camera may be made which will take fairly sharp 
pictures of all objects more than 100 ft. or so away. This 
is because objects at a greater distance than that send 
practically parallel rays into the lens and so form the 
image at the principal focus of the lens. By assuming 
that the rays entering the lens from fairly distant objects 
do converge to the principal focus, we may locate the 
focus by getting the image of a distant building on a 
screen, which will then be at the principal focus. The 
focal length of a lens is the distance from the lens to the 
principal focus. 

Experimental : 

Set the lens on a half-meter stick and move the screen 
until the most distant bright object which can be seen 

through the window is sharply 
focused on the screen. 

Take three readings, moving 
both lens and screen each time, 
recording in each case the position 
of the lens and the screen in the 
table of observations near the top 
Fig. 62. of the left-hand page. 




FOCAL LENGTH OF A CONVERGING LENS 165 
Observations 

Trial Position of Lens Position of Screen Number of Lens 

1 

2 _.__, 

3 



Make a drawing of the apparatus and show by short 
dash lines the path of the light rays before and after 
passing through the convex lens. Briefly describe the 
method of the experiment. 

In the table of calculated results at the top of the right- 
hand page, record the average distance between the lens 
and the principal focus as the focal length. 

If time permits, determine the focal length of a second 
lens, recording it in the last line of the table of calculated 
results. 

Calculated Results 

Trial 12 3 

Distance between lens and screen 

Focal length of lens No. {Average of 1, 2, & 3) 

Focal length of lens No. 



Discussion : 

Define (a) the principal focus, (5) the principal focal 
length. Why is the most distant object available selected ? 
Why is the convex lens spoken of as a converging lens ? 

Conclusion : 

The principal focal length of lens No. is 



166 LABORATORY EXERCISES 



EXPERIMENT 47 

Conjugate Foci of a Converging Lens 

OBJECT. To determine the conjugate foci of a converging lens 
and their relation to the principal focus. 

Apparatus. Double convex lens ( 10 to 15 cm. focal length); 
lens holder ; opaque screen ; screen holder ; meter stick, sup- 
ported in the slots of two wooden blocks ; opaque screen, with 
round or square window cut in it, over which is pasted a piece of 
very thin paper with ink lines ruled across it at right angles, and 
with an ink mark in one of the four spaces formed by the inter- 
secting lines ; candle or incandescent lamp to be placed behind the 
translucent window (see Fig. 61, page 161). 

Introductory : 

When a pencil of light diverges from a point and is in- 
cident on a lens, it is brought to a focus by the lens at a 
point on the axis passing through the radiant point from 
which the light came. The radiant point and the focal 
point are conjugate foci of the lens. Conjugate foci of a 
lens are points so located with reference to the lens that, 
if the object is placed at either point, the image will be 
located at the other. 

Conjugate foci may be located by determining the two 
positions between an object and a screen where a lens may 
be placed so as to form a sharp image on the screen. In 
such positions, an important relation exists between the 
distance of the object from the lens, the distance of the 
image from the lens, and the focal length of the lens. 



CONJUGATE FOCI OF A CONVERGING LENS 167 

Experimental : 

Arrange the apparatus as in Fig. 61. Then adjust the 
position of the lens so that a distinct image of the illumi- 
nated paper will be formed on the opaque screen. Is the 
image erect or inverted ? Real or virtual ? 

Measure the diameter of the object and of the image. 
Record in tabular form near the top of the left-hand page. 

Record the position on the meter stick of the object, the 
lens, and the image. 

Leaving the object and screen in position, move the lens 
until you find another position for it, at which the lens 
will again produce a distinct image. Make the same ob- 
servations as before, and record. 



Observations 






i 
Position of object 


. cm. __ 


ii 

___ cm. 


Position of lens . 





. cm. __ 


___ cm. 


Position of image . . 





. cm. __ 


--- cm. 


Diameter of object 





. cm. -- 


___ cm. 


Diameter of image . 





cm. ___ 


__ cm. 


Image — erect or inverted 









Image — real or virtual 









Number of lens .... 










Make a simple drawing, showing the arrangement of 
apparatus, and describe how it was used. 

In case you do not know the focal length of the lens, 
determine it by the method given in Experiment 46 B, on 
page 164. 

Place the table for the calculated results at the top of 
the right-hand page, and make the calculations indicated. 
All reciprocals should be worked out as decimals, the re- 
sult being carried to four decimal places. 



168 LABORATORY EXERCISES 

Calculated Results 

i ii , 

Distance of obj ect from lens . . cm. cm* 

Distance of image from lens . . cm. cm. 

1 
Object distance 

1 
Image distance 

\ + K ----- -- 

Object distance Image distance 

Principal focal length of lens . . cm. 

1 ' 

Focal length of lens 

Discussion : 

What is the relation between the diameters of the object 
and the image, and their respective distances from the 
lens ? 

Conclusion : 

Compare the sum of the reciprocals of the image and 
object distances with the reciprocal of the principal focal 
length. 






MAGNIFYING POWER OF A LENS 169 

EXPERIMENT 48 

Magnifying Power of a Lens 

OBJECT. To find the ratio of the diameter of an object viewed 
with the unaided eye to the diameter of the image seen through a 
converging lens. 

Apparatus. Two double convex lenses, of 5 and 10 cm. 
focal length respectively ; half-meter stick ; opaque screen ; lens 
holder and screen holder to slide along the meter stick ; piece of 
cardboard, 2" x 3", covered on one side with black paper, and 
with a square hole in the center 1 cm. on aside; paper metric 
scale ; ring stand with two small condenser or burette clamps, 
with cork-lined jaws. 

Introductory : 

Double convex lenses are used in certain optical instru- 
ments because the images produced by them are larger 
than the objects viewed. The ratio of the diameter of the 
image to the diameter of the object is the magnifying 
power of the lens. 

By the size of an object, we mean that apparent to the 
unaided eye. It has been found, however, that the 
majority of people obtain the most distinct vision when 
the object is 25 cm. from the eye. Accordingly, if we 
take for our object a line, it should be viewed at the dis- 
tance of most distinct vision (25 cm.). This line will 
appear longer when seen through a converging lens. The 
ratio of the apparent length of the line as seen through 
the lens to the length of the line seen with the unaided 
eye, is the magnifying power of the lens. In this man- 
ner we are comparing the diameter of an object with that 
of its image. 



170 



LABORATORY EXERCISES 




Experimental : 

(tf) Set the lens of greater focal length on some even 
centimeter division of the half-meter stick pointing 
toward the window. On the other end of the stick , place 
the screen, and move it toward the lens until the most 
distant bright object which can be seen through the win- 
dow is sharply focused on the screen. Note the distance 

between the lens and the screen 
(principal focal length) . Record 
this focal length in the table of 
observations placed near the top 
of the left-hand page. 

Place the lens horizontally 
(Fig. 63) in the jaws of a clamp 
on a ring stand, tightening the 
clamp just enough to hold the 
lens, but not enough to crack 
the glass. Adjust the clamp in 
height so that the lens is 25 cm. 
above the table top. 

Support with another clamp 
the cardboard diaphragm, so that 
its square opening is just at the principal focus of the lens. 
Place a paper metric scale on the table below the opening. 
Look down through the lens at the scale with one eye, 
while viewing the scale at the same time with the other 
(unaided) eye. Note how many millimeter divisions seen 
with the unaided eye are apparently covered by the width 
of the opening. A little practice will enable you to make 
the comparison without any straining of the eyes. 

Record the apparent width in millimeters in the table 
of observations. Measure in millimeters the actual width 
of the opening, and record. This actual width is the nuin- 




Fig. 63. 



MAGNIFYING POWER OF A LEXS 171 

ber of millimeter divisions which the unaided eye could 
see through the square opening if it were placed upon the 
scale. 

(5) Repeat the measurements of part (a), using the 
lens of shorter focal length. 

Observations 
Width of opening in diaphragm . . . _.i._. mm. 

Paet (a) Pabt (h) 

Focal length of lens . . . cm. cm. 

Apparent width of opening 

seen through lens . . . mm. mm. 

Make a drawing showing the relative position of the 
eyes, the lens, the opening in the diaphragm, and the 
metric scale when the comparison was made. Describe 
the method of making the comparison. 

The ratio between the number of millimeter divisions 
which .can apparently be seen through the opening when 
the lens is used, and the number of such divisions visible 
through the opening to the unaided eye, is the magnifying 
power of the lens. Calculate the magnifying power of the 
lenses used in (a) and (J). Record your results in the 
table of calculated results at the top of the right-hand 
page. 

Calculated Results 

Magnifying power of lens in (a) . . . times 

Magnifying power of lens in (6) . . . times 

Discussion : 

Why is the metric scale viewed at a distance of 25 cm.? 
Is the lens of shorter focal length more desirable for a 
simple magnifier than that of longer focal length ? Ex- 
plain. 



172 LABORATORY EXERCISES 

Conclusion : 

Complete the statement : 

The magnifying power of a lens is the ratio of 



EXPERIMENT 49 A 1 

The Astronomical Telescope 

OBJECT, (a) To construct and learn the operation of a simple 
astronomical telescope ; (b) to find its magnifying power. 

Apparatus. Double convex lens of short focal length (5 or 
10 cm.) ; lens holder to slide along half-meter stick; lens of long 
focal length, not over 40 cm. (a reading glass may be used) ; 
holder or clamp for supporting lens ; cardboard screen with trans- 
lucent window 1" square ; screen holder to slide along half-meter 
stick ; half-meter stick ; ring stand ; burette clamp ; strip of 
white cardboard, 20" x 3", ruled with black vertical lines 1" apart 
and ±-" thick; strip of white cardboard, about 5" x 2", with a black 
arrow 2"or 3" long drawn along the middle. 

Introductory: 

An astronomical telescope in its simplest form consists 
of two double convex lenses at the opposite ends of an 
opaque tube, with some device for varying the length of 
the tube. The lens through which the eye looks is gener- 
ally smaller than the lens at the end of the tube point- 
ing towards the object to be viewed. These two lenses, 
moreover, will be found to differ considerably in focal 
length. 

1 Experiments 49 A and 49 B are similar in method and afford similar 
training. It is recommended that only one of them be performed, unless 
there is abundant laboratory time. 






THE ASTRONOMICAL TELESCOPE 173 

To understand the operation of an astronomical tele- 
scope, we must find why two lenses are used; why the 
lenses must be so different in focal length ; what is 
meant by bringing the instrument into focus. The first 
step toward answering these questions is to determine the 
focal length of .each lens. Then by mounting them in 
suitable relative positions, we can improvise a telescope 
and determine the principles of its operation. 

Experimental : 

(a) Focal Length of the Lenses, — Take the lens of short 
focal length, which is to be used as the eyepiece of the tele- 
scope, and mount it on the end of a half-meter stick point- 
ing toward a window. Move a screen on the stick toward 
the lens, until the most distant bright object seen through 
the window is sharply focused on the screen. Is the 
image erect or inverted ? Measure on the half-meter stick 
the distance between the lens and the screen. Record 
this focal length in a table of observations near the top of 
the left-hand page. 

Mount the other lens (the objective) over one end of 
the half-meter stick, by means of the clamps and ring 
stand, and determine its focal length in a similar manner, 
and record. 

(&) Use of the Lenses. — Leaving the objective focused 
on the screen, mount the eyepiece on the meter stick, on 
the other side of the screen, so that the centers of the two 
lenses are on the same horizontal line. (Fig. 64.) 

Looking through the eyepiece, move it along the stick 
until you can see distinctly the image thrown on the screen 
by the objective. Record the distance of the eyepiece from 
the screen. Does the image viewed through the eyepiece 
appear larger or smaller than the image that the unaided 
eye can see on the screen ? 



174 



LABORATORY EXERCISES 



Leaving the lenses undisturbed, remove the screen. 
Again look through the eyepiece. Can you see the image 
of the distant object ? Record the distance between the 
objective and the eyepiece. 

(c) Focusing. — Shift the meter stick in the clamp a few 
centimeters. Move the eyepiece until you can see dis- 
tinctly through it the image of the distant object. This 

is the method of 
focusing the eye- 
piece of a tele- 
scope on the im- 
age of a distant 
object projected 
through the ob- 
jective. Record 
the distance be- 
tween the objec- 
tive and the eye- 
piece. 

(<#) Magnify- 
trig Power. — On 
the most conven- 
ient and distant 
wall of the room, 
place as an object, a black arrow on a strip of white card- 
board. The arrow should be in the same horizontal plane 
as the centers of the lenses of the telescope. 

Focus the telescope on the black arrow. Have another 
student stand at the distant wall and move the scale with 
the black ruled lines down toward the arrow while you 
are looking through the telescope. By using both eyes 
at the same time, you will be able to see how many divi- 
sions of the scale are equal to the apparent length of the 
arrow, as seen through the telescope. Measure the real 




Fig. 64. 



THE ASTRONOMICAL TELESCOPE 175 

length of the arrow in divisions of the ruled scale. 
Record both measurements in the table of observations. 
How many times is the length of the arrow magnified by 
the telescope ? Record this magnifying power in the table 
of calculated results. 

Observations 

Part (a) Focal length of eyepiece . . . cm. 
Focal leiigth of objective . . . cm. 
Part (5) Distance of eyepiece from screen . cm. 
Distance between objective and eye- 
piece cm. 

Part (c) Distance between objective and eye- 
piece cm. 

Part (d) Actual length of arrow .... divisions 
Apparent length of arrow through 

telescope divisions 

Make a simple outline drawing, showing the arrange- 
ment of the lenses in your telescope. Describe briefly 
the steps you took in each part of the experiment. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

Sum of focal lengths of objective and eyepiece cm. 

Magnifying power of telescope times 

Ratio of focal length of objective to 

focal length of eyepiece .... = 

cm. 1 

Discussion °. 

Why is a lens of long focal length used as the objective 
of an astronomical telescope ? What is the purpose of the 
eyepiece (the lens of short focal length) ? Compare the 



176 LABORATORY EXERCISES 

observed magnifying power with the ratio of the focal 
lengths. 

Conclusion : 

Complete this statement: 

When an astronomical telescope is focused on a distant 
object, the distance between the objective and the eye- 
piece is equal to the 



EXPERIMENT 49 B 

The Compound Microscope 

OBJECT. To construct a compound microscope and to determine 
its magnifying power. 

Apparatus. Two double convex lenses of short focal length 
(about 5 cm.) ; lens holder and screen holder, to slide along a 
half-meter stick ; half-meter stick ; piece of cardboard, with a 
black arrow 1 cm. in length drawn on the middle of one side; 
ring stand with two cork-lined burette or small condenser clamps ; 
paper metric scale. 

Introductory : 

The compound microscope in its simplest form consists 
of two converging lenses of short focal length, mounted 
a suitable distance apart and usually at the ends of an 
opaque tube. The objects viewed with the microscope 
are small ones and they are placed under the lens known 
as the objective and just beyond its principal focus. The 
image formed by the objective is viewed through the other 
lens (the eyepiece). The magnifying power depends on 
the focal lengths of the two lenses used and also on the 
distance between them. 



THE COMPOUND MICROSCOPE 



177 



The magnifying power may be defined as the ratio 
between the diameter of the image seen through the 
microscope and the diameter of the object viewed with the 
unaided eye. We shall view a little ruled arrow through 
the microscope, while the other eye is looking at a metric 
scale placed beside the arrow. 

Experimental : 

(a) Focal Length of the Lenses. — Take one of the lenses 
for the eyepiece and mount it on a half-meter stick near 
the end pointing toward a window. Move a screen on 
the meter stick toward the lens until the most distant 
bright object to be seen through the window is sharply 
focused on the screen. Then measure on the meter stick 
the distance between the lens 
and the screen. Record this 
focal length in the table of 
observations near the top of 
the left-hand page. 

Determine similarly the 
focal length of the other lens 
(the objective) and record 
the distance in the table. 

(6) Construction. — Place 
on the base of the ring stand 
a piece of white cardboard 
with its drawing of a little 
arrow for the object. 

Carefully mount each of 
the lenses in the cork-lined 
jaws of a small clamp and arrange on the ring stand as 
shown in Fig. 65. The objective should be fixed above 
the object at a height greater than the focal length of 
that lens, but at less than twice the focal length. 




Fig. 65. 



178 LABORATORY EXERCISES 

Looking through the eyepiece, move that lens up and 
down until a sharply defined, black image of the little 
arrow is seen. Is the image real or virtual? In what 
two respects is the image different from the objec 
Measure the height of the objective above the object and 
the vertical distance between the centers of the two 
lenses. Record these distances in the table. Leave the 
microscope focused for part (c). 

(^eO ^Magnifying Power. : — Place a paper metric scale 
near the little arrow and parallel to it. With one eye look 
through the microscope at the arrow, while, at the same 
time, with the other (^unaided^ eye you are viewing the 
metric scale. Slide the metric scale so that you can 
measure the length of the arrow, as it appears through 
the microscope. The divisions of the scale should not 
look magnified. Record the apparent length of the ar- 

w in millimeters. Measure the actual length of the 
arrow with the scale and record it in the table. What 
is the apparent magnifying power of your micro.-; 

Observations 

F ~~ - -' .v . . . . , cm. 

F ft t ....... 

D em. 

I> enters ro- 

'> - us- i : em. 

Ckt ■ limin- 


A ■ • ^ mm. 

A-:tu.i. ' mm. 

ople diagram showing the relative positions 
of the eye, the two lenses, and the object. Describe, 



DISPERSION OF LIGHT BY A PRISM 179 

with reference to the drawing, the work in (6). De- 
scribe also how you determined the magnification in (c). 

Calculated Results 
Magnifying power of microscope .... times. 

Discussion : 

Why should not the object be placed at the principal 
focus of the objective ? When the object is beyond the 
principal focus, but not distant twice the focal length 
from the objective, how does the image compare in size 
with the object ? Is this image real or virtual ? Upon 
what is the eyepiece focused ? Compare the distance 
between the lenses with the focal length of the eyepiece. 
Explain how this distance affects the magnifying power 
of the microscope. 

Conclusion : 

State the essentials in the construction of a compound 
microscope. How must the eyepiece and the objective 
be placed to secure magnification ? 

EXPERIMENT 50 

Dispersion of Light by a Prism 

OBJECT. To observe the effect of a triangular glass prism on a 
beam of white light. 

Apparatus. Triangular glass prism, 60° ; opaque covering for 
the upper half of laboratory window, with a slit 6 in. x f in. ; 
second opaque covering, with |" slits arranged as in Fig. 67. 

Introductory : 

When we look through glass prisms, such as are some- 
times hung from the bottom of a lamp shade, we notice that 



180 



LABORATORY EXERCISES 



the outlines of objects show a fringe of color. When the 
sun begins to shine before the rain stops falling, a rain- 
bow, consisting of a band of the same 
colors seen through the glass prism, 
appears in the sky. In both of these 
cases white light has been broken up 
into colors, by passing through a trans- 
parent medium more dense than air. 




Fig. 66. 



Experimental : 

(#) The upper part of the laboratory 
window will have an opaque covering, 
in which a narrow slit has been cut. 
Take a position from which the sky can 
be seen through the slit. Close one 
eye and, holding the prism in front of the other with one 
edge pointing toward the slit, rotate the prism slowly 
until the slit appears as a band of color. Name the colors 
distinctly seen, in the order in which they appear to you. 
Since the different col- 
ors appear' in different 
positions, what must be 
true of the amount of 
bending of each ? No- 
tice carefully the posi- 
tion of the faces of the 
prism and see if you can 
determine which of the 
colors is most refracted, 
(ft) On the upper half 
of another laboratory 
window there has been 
placed an opaque covering with three pairs of slits ar- 
ranged as in Fig. 67. Holding the prism in front of 













<-2-> 








JH 






*1* 











Fig. 67. 



FIXED POINTS OF A THERMOMETER 181 

the eye as before, look at the slits. Note the overlap- 
ping of the spectra. Find a case in which there is vis- 
ible a color not evident in the spectrum as seen in (a). 
What colors by their overlapping produced this new 
color ? Can you find another distinct color formed by 
the combining of the light rays of two different colors ? 
In which case do you find a streak of white produced by 
the overlapping spectra. What colors combined to form 
this whitish light ? 

Make one sketch showing how the prism was held in 
front of the eye, and two diagrams showing the arrange- 
ment of the slits on the opaque coverings. Describe the 
experiment with reference to these drawings, stating the 
results in each case. The description may be shortened 
by the use of further diagrams in which the colors of the 
spectra are mapped. 

Conclusion : 

What happens to a ray of white light in passing through 
a glass prism ? How may other colors be formed from 
the primary colors of the solar spectrum ? 



EXPERIMENT 51 

Fixed Points of a Thermometer 

OBJECT. To test the boiling and the freezing points on a 
mercury thermometer. 

Apparatus. Steam boiler with bent glass delivery tube ; 
1-hole rubber stopper tightly fitting the top of the boiler chimney; 
hydrometer jar; thermometer, — 10° to 110°C. ; glass funnel; 
cylindrical or other jar to support funnel ; burner ; supply of 
cracked ice. 



1S2 



LABORATORY EXERCISES 



Introductory : 

A long- while ago it was noticed that when pure water 
was boiled at the top of a high mountain, it was not so 
hot as when pure water was boiled near the sea level. 
This is because the pressure of the air is less at the moun- 
tain top. The boiling point of pure water, under standard 
conditions of barometric pressure, is 100° C. We wish to 
test a thermometer to determine whether its 100° mark is 
correctly placed, and also to rind its error, if any. We 
shall also test its zero graduation, which should mark the 
temperature of water in the process of freezing, or of ice 
in the process of melting. 

Experimental : 

Place the tabular form for observations near the top of 
the left-hand page. Record all readings as soon as made. 

(<0 Freezing Point. — Fill a funnel about 
half full of cracked ice and support it in a 
jar (Fig- 68). Insert the thermometer in 
such a way that the ice will be packed around 
the bulb and nearly to the zero of the scale. 

After the mercury lias remained stationary 
for at least rive minutes, take the reading of 
the thermometer to tenths of a degree. The 
difference between this reading and the zero 
of the scale is the freezing point error of the 
instrument. No allowance need be made for 
the atmospheric pressure. 

State the correction for your thermometer 
as — or +, according as the freezing point 
was indicated too high or too low. This 
correction should be added algebraically to 
all readings near the freezing point taken 




Fig. 68. 



with this thermometer. 



FIXED POINTS OF A THERMOMETER 



183 



J 



rJ 



£l 



(5) Boiling Point. — To test the boiling point of the 
thermometer, see that the chimney is on the boiler and the 
thermometer adjusted so that the 100° mark is just above 
the stopper. The upper tube of the boiler should be 
open; the lower one closed. The bulb of the thermometer 
should not dip into the water in 
the boiler, which is half filled 
with water. Boil the water until 
the reading of the thermometer 
remains stationary for at least 
two minutes. Then take a read- 
ing of the thermometer, estimat- 
ing to tenths of a degree, and 
record. 

(<?) Next determine the effect 
on the boiling point when the 
pressure is increased. To the 
upper side tube attach the bent 
glass tube so that it points down- 
ward. When steam is escaping 
vigorously, immerse the long glass tube in a jar of water, 
so that its free end reaches nearly to the bottom of the jar 
(Fig. 69). Observe the temperature when the mercury 
becomes steady, so as to determine the effect of increased 
pressure on the boiling point. 



Fig. 69. 



CAUTION. As soon as the reading has been made, withdraw the 
loDg tube from the jar so that hot water may not crack the jar. Turn 
out the flame under the boiler. 



Calculation of the True Boiling Point. — The instructor 
will give you at this point the barometer reading of the 
day. It has been found that a difference of a millimeter 
in pressure makes a difference of 0.037° C. in the boiling 
point. Then for every millimeter of the barometer read- 



184 LABORATORY EXERCISES 

ing in excess of 760 mm., add 0.037° C. to 100° C., or sub- 
tract 0.037° C. for every millimeter of barometer pressure 
less than 760 mm. This gives the true boiling point of 
water under existing barometric conditions. 

The difference between this and the calculated boiling 
point will be the error of your thermometer at the boiling 
point. State the correction necessary to bring your ther- 
mometer to the true boiling point as + or —-, according as 
the boiling point was indicated too low or too high. This 
correction is added algebraically to indicated temperatures 
near the boiling point whenever the thermometer is used. 

Observations 

Number of thermometer 

Reading in melting ice . ° C. 

Reading in free steam ° C. 

Reading in steam under pressure .... ° C 

Barometer reading mm. 

Make sectional drawings of your apparatus, showing 
how it was used. Write a simple description of how you 
did each part of the experiment. 

Place the table of calculated results at the top of the 
right-hand page. 

Calculated Results 

True boiling point for to-day ° C. 

Boiling point error of the thermometer ... ° C. 
Correction for thermometer at boiling point 

(+ or -) ° a 

Freezing point error of thermometer ... ° C. 
Correction for thermometer at freezing point 

(+ or -) ° O. 



PHENOMENA OF BOILING 185 

Discussion : 

What is the general effect of pressure on the boiling 
point ? How could you properly graduate a finished blank 
or ungraduated thermometer ? 

Conclusion : 

The corrections for thermometer No are ° C. 

(+or-) 

at the freezing point, and ° C. at the boiling point. 

(+or-) 



EXPERIMENT 52 

Phenomena of Boiling 

OBJECT. To observe what changes occur during boiling and 
the effect of a dissolved substance on the boiling point. 

Apparatus. Distilling flask, 150 cm, 3 ; ring stand with 
wire gauze supported on ring ; small clamp ; perforated flat cork 
(1"); cork stopper to fit neck of flask and perforated to admit 
thermometer; beaker; elbow tube and rubber connections ; ther- 
mometer reading to 100° C; Bunsen burner ; if part (d) is to be 
done, — boiler and hydrometer jar ; short pieces of glass tubing or 
rod. 

Material. Coarse salt. 

Introductory : 

Our first ideas of boiling were probably obtained from 
watching the teakettle at home. We knew that the cover 
should be put on the kettle if the water were to heat 
quickly. We have seen the lid rise a little and then bump 
back, until finally steam issued from the spout and a sing- 
ing noise was heard. 



186 



LABORATORY EXERCISES 



All these familiar sights and sounds are the phenomena 
of boiling, and boiling means simply the disturbances and 
changes that occur during the transformation of a liquid 
into a gas. In a glass flask all these phenomena may be 
readily observed, and no matter how many times we may 
see the operations, they lose none of their first fascination. 

Experimental : 

Arrange the apparatus as in Fig. 70. The position of 
the cork stopper held in the clamp may have to be adjusted 
from time to time so that the thermometer scale may be 

read. The perforated flat 
cork is slipped over the 
thermometer and then ad- 
justed in position so that 
it forms a cap resting on 
the top of the neck of the 
flask. The thermometer 
should slip easily through 
this cork. 

Record all observations 
as soon as made in a tabu- 
lar form near the top of 
the left-hand page (see 
page 188). 

(a) Have the flask a 
little less than half full 
of fresh water. Heat the 
flask with a small flame, 
noting where bubbles first form, the size of the bubbles, 
and what becomes of them (Observation 1). When this 
first bubbling ceases, remove the flame. Slowly lower the 
thermometer so as to immerse the bulb in the liquid. 
Note the temperature (Observation 1). Is the water at 




Fig. 70. 



PHENOMENA OF BOILING 187 

its boiling point ? What makes you think that these first 
bubbles might be air bubbles ? 

Raise the thermometer from the water and resume the 
heating. Note where the bubbles begin to form after a time 
and what happens to them as they proceed toward the sur- 
face (Observations 2 and 3). Take the temperature of the 
top layer of water (Observation 2) ; also the temperature 
of the water near the bottom of the flask (Observation 3). 
Explain the behavior of the first bubbles in this second heating. 

Raise the thermometer so that its bulb stands just be- 
low the opening from the neck to the delivery tube of the 
flask. Watch the formation and action of the bubbles as 
the heating continues, increasing the flame if necessary 
(Observations 4, 5, and 6). When steam is escaping 
freely from the flask, note the thermometer reading (Ob- 
servation 4). Explain. Then take the temperature of the 
water near the top (Observation 5) and also at the bottom 
of the flask (Observation 6). What happens to the steam 
passing out the delivery tube? 

In case your flask bumps at any time during the heating, 
see what happens just before the moment of the bumping. 

(5) Incline the distilling flask, and let two short pieces 
of glass tubing or glass rod slide down the inside of the 
neck. Continue the heating, noting where bubbles form 
(Observation 7). What is the effect on the rate of boil- 
ing ? What is the effect of introducing the glass pieces on 
the amount of heated surface ? 

Glass beads are often used to prevent bumping, as well 
as to save time in laboratory distillations. 

(<?) Remove the flask from the flame, and after inclin- 
ing it, slide in about a dozen pieces of coarse salt. Slowly 
add water until the flask is nearly half full again and, 
after wiping the outside dry, replace the flask on the gauze. 
Clamp the thermometer so that its bulb is immersed. 



188 



LABORATORY EXERCISES 



Note the temperature of the liquid when it begins to 
boil freely 1 (Observation 8). Raise the thermometer so 
as to take the temperature of the vapor (Observation 9). 
Taste the condensed liquid coming from the delivery 
tube. Is it salty? 

(<T) In case Part (e) of Experiment 51 (page 183) has 
not been performed, determine the effect on the boiling 
point when the pressure is increased. Arrange the ap- 
paratus as represented in Fig. 69 on page 183. Then fol- 
low the directions given in the first paragraph of (<?), 
on that page. 

CAUTION. As soon as the reading has been made, withdraw the 
long tube from the jar, so that the hot water may not crack the jar. 
Turn out the flame under the boiler. 

Observations 



Number of 


Position of 


Tempera- 


Where Bubbles Form and 


Observation 


Thermometer Bulb 


ture 


Their Behavior 


(a) Water 








1 








2 








3 








4 








5 








6 








(b) Pieces 








of glass 








7 
(c) Salt solution 














8 








9 








(d) Increased 








pressure 








10 









1 Note to Instructor. While waiting for the boiling to occur in Part 
(c), the students should be directed to work on their laboratory note- 
books. 



PHENOMENA OF BOILING 189 

Make a drawing of your apparatus. Complete the de- 
scription of how the experiment was done by statements 
supplementing the information given in the table of ob- 
servations. 

Discussion : 

Answer under this heading the italicized questions 
occurring in the experimental directions. 

What change of state occurs in the vaporization (boil- 
ing) of a liquid? in the condensation of a vapor? 

Conclusion : 

Complete these statements : 

A liquid boils best in a flask with a ] ,1 surface. 

u ( rough j 

The boiling point of a water solution is than that of 

pure water. The boiling point is by an increase of 

pressure. 



190 LABORATORY EXERCISES 

EXPERIMENT 53 

Coefficient of Linear Expansion 

OBJECT. To determine the coefficient of linear expansion of a 
given material. 

Apparatus. Any form of linear expansion apparatus, the 
essentials being : a tube or rod, so mounted that one end is 
clamped firmly and the other is free to move ; if a rod is used, 
an outer tube to serve as a steam jacket; a steam boiler, with 
rubber tubing to connect it with the steam jacket or tube ; Bunsen 
burner ; thermometer ; meter stick ; lever and scale, or microm- 
eter screw, for magnifying the elongation. 1 

Introductory : 

The fact that bodies expand when they are heated is 
very familiar. The space left between the ends of the 
rails on a railroad is designed to allow for the difference 
in length in winter and summer. Although the propor- 
tional expansion is very small, the total change in length 
of a long structure may be considerable. In order to cal- 
culate the total elongation of any body when heated, it 
is necessary to know the change produced in a unit length 
by a change in temperature of one degree. This increase 
in length per unit length per degree Centigrade is called 
the coefficient of linear expansion of the material. As the 

1 As different schools are supplied with different forms of apparatus 
for this experiment, it was not considered advisable to confine the experi- 
mental directions to any one form. The authors would recommend to 
schools making their own apparatus or purchasing new apparatus, that 
the expansion of a tube rather than that of a rod be measured, as this 
greatly simplifies the apparatus. If the tube is just a meter long, and 
the magnifying ratio of the lever or screw is an even one, as 1 to 20, 1 to 
50, or 1 to 100, calculations will be greatly simplified and the pupil will 
see the result much more clearly. 



COEFFICIENT OF LINEAR EXPANSION 191 

total increase in length of such a specimen as can be used 
in the laboratory is very small, a magnifying lever, having 
a known ratio between the arms, or a micrometer screw, 
is commonly employed to make possible the accurate cal- 
culation of the total elongation. 

Experimental : 

The length of the specimen furnished is to be directly 
measured in centimeters and tenths with a meter stick. 
Care should be taken in this measurement and in the ad- 
justment of the apparatus for the zero reading, to handle 
the specimen as little as possible, so that its initial tem- 
perature may remain that of the room. This temperature 
is obtained by reading a thermometer wdiich has been in 
contact with or very close to the specimen for some time. 

The specimen is then mounted as directed by the in- 
structor, care being taken that it is free to move only at 
the end provided with the device for obtaining the amount 
of elongation. The position of the pointer, or the microm- 
eter head, at the room temperature is then observed to 
tenths of the smallest division, and recorded. The room 
temperature is also recorded. 

Steam is next passed through the tube, or through the 
jacket surrounding the rod, for at least ten minutes. If 
a steam jacketed rod is used, the temperature of the rod 
may be taken as that of a thermometer whose bulb is 
inside the jacket in contact with the rod. If a tube is 
used, through which the steam directly passes, the tem- 
perature may be taken as the boiling point of the day, 
which will be furnished by the instructor. 

With the lever form of apparatus, it is only necessary 
to take the final reading of the pointer and determine the 
ratio of the lever arms. When a micrometer is used, the 
screw should be turned back from the end of the rod im- 



192 LABORATORY EXERCISES 

mediately after taking the reading at room temperature, 
and, after the tube has reached the temperature of the 
steam, the screw is again brought in contact with the end 
of the tube and the reading taken and recorded. Steam 
should be passing freely when the final readings are 
taken. 

As soon as the readings have been taken, the steam 
supply should be discontinued, so that the specimen may 
cool to room temperature as rapidly as possible, and so be 
ready for a repetition of the experiment if necessary. 

All readings taken directly from the apparatus are to 
be recorded in tabular form near the top of the left-hand 
page. 

Observations 

Initial length cm. 

Initial scale reading cm. 

Final scale reading cm. 

Ratio of lever arms . . 

Room temperature ° O. 

Steam temperature ° O. 

Make a simple outline drawing of the apparatus, and 
write a brief description of the method employed in the 
experiment. 

From the readings obtained, the total elongation, the 
change in temperature, and the expansion in centimeters 
per degree Centigrade per centimeter can be obtained 
by calculation. The results should be entered in tabular 
form at the top of the right-hand page. 

Calculated Results 

Difference in scale readings cm. 

Total expansion 8 cm. 



COEFFICIENT OF CUBICAL EXPANSION 193 

Difference in temperature ° C. 

Expansion per degree 0. cm. 

Expansion per degree 0. per centimeter (linear 
coefficient) 

Discussion : 

Explain the method of calculating the actual expansion 
from the scale readings, if the lever apparatus was used. 

If the micrometer apparatus was used, explain the 
method of obtaining readings with the micrometer. 

Conclusion : 

The coefficient of linear expansion of is __♦___. 



EXPERIMENT 54 

Coefficient of Cubical Expansion 

OBJECT. To determine the coefficient of cubical expansion of 
mercury, relative to glass. 

Apparatus. Specific gravity bottle, 25 cm. 3 , having a stopper 
with a capillary hole ; x ring stand with one ring and wire gauze ; 
thermometer; pipe-stem triangle, with the wire ends doubled 
under; beaker large enough to permit the bottle, resting on the 
triangle, to be immersed to a point above the bottom of the stopper ; 
Bunsen burner; balance; weights; one funnel for the class, as 
described in footnote on page 194. 

Material. Mercury. 

Introductory : 

The tubes of alcohol thermometers are larger than those 
of mercury thermometers, because alcohol has a greater 

1 The use of the specific gravity bottle in this experiment was suggested 
by Mr. Charles H. Slater. 



li)4 LABORATORY KXKRCISKS 

pate of expansion than mercury. Both alcohol and mer- 

OUry expand more than glass; otherwise the expansion of 
the glaSS bull) and stein would cause the liquid column in 
tlu i stem to fall instead of rise. Since mercury is often 
contained in glass vessels, as in the thermometer, barome- 
ter, and other pieces of apparatus, it is important to know 
the relative rale of expansion of the two. 

A bottle completely filled with a known weight of mer- 
cury is heated through a measured change of temperature, 

and the weight of the mercury which overflows is found. 
As weights of the same substance are proportional to the 

corresponding volumes, it is easy to calculate the propor- 
tion of the Original volume by which the mercury expands 
for each unit, change of temperature. This quantity is 
the coefficient of cubical expansion of mercury. As the glass 

bottle has been heated to the same temperature as the 

mercury, the expansion measured is relative to the expan- 
sion of the glass, and so the coefficient obtained is relative 

to glass. 

Experimental : 

A specific gravity bottle, having a perforated stopper, 
is filled with mercury under the direction of the instructor. l 
("are must be taken in filling to see that no air is left 
under the stopper and that the capillary tube in the Stop- 
per is completely filled. The bottle should be bandied by 

the neck, to prevent the heat of the hand from forcing 

1 The mercury may wed bv opntained in n Funnel, having a jet tube 
attached by soft rubber tubing, with a screw compressor on the tubing. 
a porcelain dish is plaoed beneath it to oatoh the overflow, At the close 
of the experiment, the pupils may pour off most i^ the water from their 

beakers, anil then pour the mereurv ami remaining water baek into the 
funnel. The separation of water from mereurv will then be eompara- 
tivelv easy. 



COEFFICIENT OF CUBICAL EXPANSION 



195 



any mercury out. Tin; bottle and its contents are then 
weighed at room temperature, and both weight and tem- 
perature recorded in tabular form near the top of the left- 
hand page. 

The bottle is next placed on a pipe-stem triangle, in a 
beaker on a ring stand, and water added until it has 
reached the level of the bottom of 
the stopper in the bottle (Fig. 71). 
The water is heated with a Bunsen 
burner until it boils, and kept at a 
boiling temperature for 5 minutes. 
While it is boiling, the temperature 
of the water is taken with a ther- 
mometer and recorded. Observe 
what happens to the mercury in 
the bottle. 

The beaker is now removed from 
the ring stand. By dipping out 
hot water and adding cold water, 
the temperature of the water is 
brought approximately to that of 
the room. Care must be taken 
during this operation to avoid get- 
ting any water into the specific 
gravity bottle. The bottle is now removed from the 
water, carefully dried, and again weighed. 

After being weighed, the bottle is returned to the 
instructor, still containing the mercury. The greater 
part of the water in the beaker is to be poured off, with- 
out losing any mercury. The remaining water and mer- 
cury is to be disposed of as the instructor may direct. 

The readings taken should be entered in a tabular form 
near the top of the left-hand page. 




Fig. 71. 



196 LABORATORY EXERCISES 

Observations 

Weight of empty bottle g. 

Weight of bottle filled with mercury, initial . . g. 

Weight of bottle with mercury, final .... g. 

Initial (room) temperature °C. 

Final (boiling) temperature ...... °C. 

A simple drawing of the apparatus and a brief descrip- 
tion of the operations in the experiment should follow the 
table of observations on the left-hand page. 

At the top of the right-hand page, place the following 
table of calculated results, making the required computa- 
tions on the page immediately beneath the table. 

Calculated Results 

Initial weight of mercury (a) g. 

Weight of mercury lost by expansion (b) . . g. 
Change in temperature (c) °(7. 

Loss by expansion per degree ( d = -] . . . g. 
Coefficient of cubical expansion I - ) 

Discussion : 

Explain clearly how the loss in weight per degree, 
divided by the original weight, gives the coefficient of 
cubical expansion. 

Conclusion : 

The coefficient of cubical expansion of mercury relative 
to glass is . 






INCREASE IN VOLUME AT CONSTANT PRESSURE 197 
EXPERIMENT 55 

Increase in Volume at Constant Pressure 

OBJECT. To find the relation between the increase in the volume 
of a gas and the increase in temperature causing the change, when 
the pressure remains constant. 

Apparatus. Steam boiler with chimney, having the lower 
side outlet closed with a rubber tube and a screw compressor ; 
Charles' law tube, Waterman form ; thick cardboard square, 
3" x 3", perforated to admit Charles' law tube ; narrow jar or 
cylinder about 8" high ; thermometer ; ring stand and clamp ; 
Bunsen burner. 

Material. Finely cracked ice, or snow. 

Introductory : 

The expansion of solids by heat is a familiar fact. The 
rate at which metals expand for each degree of tempera- 
ture has been carefully studied. It has been found that 
each metal has its characteristic rate (coefficient of ex- 
pansion). With gases, however, the rate is the same for 
them all, but its determination is made more difficult by 
the fact that the volume of a gas is also affected by atmos- 
pheric pressure. The effect of this pressure in the ex- 
pansion of solids is so slight that it is disregarded in the 
determination of their coefficient of expansion. 

As the atmospheric pressure seldom varies much in a 
short time, the effect of an increase of temperature on the 
volume of a gas is found by measuring the volume at two 
different temperatures and under atmospheric pressure. 
The two most convenient temperatures for the determina- 
tion are 0°C. and 100° C, respectively, as these tempera- 
tures are easy to obtain with ice and steam. The gas is 
confined in a Charles' law tube by means of a mercury 



198 



LABORATORY EXERCISES 



plug, which is free to move as the volume of the gas 
changes. 

The relation between volume and temperature under 
constant pressure is most conveniently expressed with 
reference to the absolute scale of temperature. On this 
scale the zero corresponds to — 273° C. The Centigrade 
zero is equivalent to 273° A. To change Centigrade 
temperatures to absolute temperatures, add 273° algebrai- 
cally. 

Experimental : 

With the steam boiler half full of water, light the 
burner underneath, and screw on the chimney. The 
lower outlet for the escape of steam should be closed by a 

screw compressor 
on a rubber tube. 
While waiting for 
the water to boil, 
determine the vol- 
ume of inclosed air 
at 0° C. as directed 
in (a). 

(a) The mer- 
cury index in the 
Charles' law tube 
should stand at 
about the center of 
the graduated scale. 
Insert the tube, 




Fig. 72. 



with its scale, in a jar containing finely cracked ice, 
or snow, so that the mercury index is a short distance 
above the surface of the ice. Note the descent of the 
index as the inclosed air contracts. When no further 
contraction occurs and the inclosed air is all surrounded 



INCREASE IN VOLUME AT CONSTANT PRESSURE 199 

by melting ice (Fig. 72, B), take the reading of the index 
on the graduated scale. Since the tube is of uniform 
diameter, the length of the inclosed column of air may be 
taken as the measure of its volume. Record the reading 
in a tabular form near the top of the left-hand page. 

(b) Remove the air tube from the ice, and allow it to 
stand five minutes or so in the air to regain the room 
temperature. Then slowly slip the tube with its scale 
through the cardboard cover on top of the chimney of the 
boiler. The mercury index should be just visible above 
the cardboard (Fig. 72, 0). When the column of in- 
closed air has been brought to the temperature of the 
steam, the index will become stationary. Clamp the tube 
in this position, with the index just above the cardboard, 
then read and record the position of the index on the scale. 
Remove the tube from the boiler. 

Insert the bulb of a thermometer in the steam within 
the chimney. When the mercury becomes stationary, 
read and record the temperature. 

Observations 

Length (volume) of inclosed air at 0° O. . . cm. 
Length (volume) of inclosed air at steam 

temperature cm. 

Temperature of the steam ° C. 

Make simple drawings, showing the Charles' law tube 
in position at each of the two temperatures. Briefly de- 
scribe the experimental method. 

Change the Centigrade temperatures to absolute tem- 
peratures, and record in a table of calculated results placed 
at the top of the right-hand page. 

Find (a) the increase in volume of the air ; (b) the 
fraction of the original volume which this increase is, 



200 LABORATORY EXERCISES 

expressing the result to three decimal places ; (c) the 
fractional (decimal) increase in temperature over the initial 
absolute temperature. Record these in the table. 

Calculated Results 

0° Centigrade ° absolute 

° Centigrade ° absolute 

Increase in length (volume) of inclosed 

air cm. 

Fractional increase in volume . = 

Fractional increase in temperature = 

Discussion : 

Why was the inclosed gas regarded as being under 
constant pressure ? Compare the two decimals represent- 
ing the fractional increases in volume and in temperature 
(absolute scale). What would be the fractional increase 
in volume for one degree? for twenty degrees? What 
would be the fractional decrease in volume when the gas 
was cooled ten degrees ? In each case assume the original 
temperature to be 0° C. 

Conclusion : 

Complete this statement: 

Under constant pressure, the volume of a gas is to 

its temperature on the scale. 



INCREASE IN PRESSURE AT CONSTANT VOLUME 201 
EXPERIMENT 56 

Increase in Pressure at Constant Volume 

OBJECT. To find the relation between the increase in pressure 
of a gas and the increase in temperature causing this change, when 
the volume of the gas remains constant. 

Apparatus. Charles' law tube (Hall and Bergen form) ; 
glass condenser with inner tube removed ; 1-hole cork stopper to 
fit opening at one end of condenser tube and solid cork for other 
end ; ring stand with condenser clamp ; ring stand with small 
clamp for raising free end of Charles' law tube ; steam boiler with 
cap ; rubber tubing to connect steam boiler with condenser tube ; 
tubulated ice tray with 1-hole cork to fit ; burner; meter stick ; 
beaker. 

Material. Cracked ice, or snow. 

Introductory : 

If a hot fire is maintained under a steam boiler when 
the engine is not running, the steam pressure increases 
and, if it were not for the safety valve, the boiler would 
burst. When a tea kettle begins to boil, the pressure of 
the steam lifts the lid. In both of these cases a gas, 
steam, is heated in such a way as to prevent it from 
expanding. In our experiment, a certain amount of air 
will be confined in a tube at the temperature of melting 
ice ; it will then be heated to the temperature of steam, 
but its volume will be kept the same by increasing the 
pressure upon it. From our results we may reach a con- 
clusion regarding the relation between the temperature of 
a gas and its pressure, when the volume is kept constant. 
The Centigrade temperatures given by our thermometer 
will be changed to absolute temperatures by adding 273° 
algebraically to the Centigrade reading. 



202 LABORATORY EXERCISES 

Experimental : 

CAUTION. Do not allow the open end of the Charles' law tube to 
get below the horizontal position, or the mercury may run out. 

See that the steam boiler is half full of water, the cap 
in place, and the steam outlet at the side open. Connect 
with rubber tubing the steam outlet of the boiler with the 
steam inlet of the steam jacket (condenser tube). Place 
a beaker beneath the outlet tube of the condenser, Fig. 74, 
to catch any condensed steam. Light the burner under 




Fig. 73. 

the boiler, so that there will be a supply of steam ready 
for the steam jacket. 

(a) Pass the closed end of the air tube through the 
cork of the ice tray and cover this portion of the tube 
with finely cracked ice. When the inclosed air column 
no longer contracts, adjust the position of the tube in the 
stopper, so that the mercury in the tube extends just to 
the outer end of the stopper ((7, Fig. 73). The other 
end of the mercury column should be at the same height 
above the table top as the mercury at (7, so that the 
volume, (7Z), of inclosed air will be at atmospheric pressure. 
The necessary elevation may be obtained by the use of a 
small piece of glass tubing ((7, Fig. 73). 

Measure the distance from O to JS, the nearer end of the 
rubber connection, and record in the table of observations 
near the top of the left-hand page. 

(5) Remove the air tube with its stopper from the ice 
tray, and fit it into the steam jacket (Fig. 74) so that the 
distance BO is just the same as with the ice tray. 



INCREASE IN PRESSURE AT CONSTANT VOLUME 203 

Support the outer end of the air tube in a movable 
clamp on a vertical support. As the air in the tube 
expands, keep raising the level of the outer tube (AB, 
Fig. 74), so that the inner end of the mercury column 
extends just to G. By this means the volume of the 
inclosed air is kept the same as the volume of the air 
which was measured at 0°C. 

In order to keep this volume of air constant, it has been 
necessary to increase the pressure upon it by raising a 




Fig. 74. 



portion of the mercury column. The increase in pressure, 
in millimeters of mercury, is the difference between the 
height of the outer and the inner ends of the mercury 
column. Determine these vertical distances above the 
table top and record them in the table. Also read the 
barometer and record the reading. 



Observations 

Part (a) Temperature 0° C. (melting ice) 

Length BC mm. 

Pressure of inclosed air (Barometer reading) mm. 

Part (b) Barometer reading mm. 

Height of outer end of mercury above table top mm. 

Height of inner end of mercury above table top mm. 



204 LABORATORY EXERCISES 

Make simple drawings, showing the arrangement of the 
air tube at each of the two temperatures. Briefly describe 
the experimental method, with particular reference to the 
means of keeping the volume constant. 

Calculate the boiling point of water (temperature of 
steam) at the observed barometric pressure. This is done 
by adding to 100° C, 0.037° for each millimeter of baro- 
metric pressure above 760 mm., or subtracting the same 
amount from 100° C. for each millimeter below 760 mm. 
Record this temperature of steam in a table of calculated 
results at the top of the right-hand page. Change the 
two Centigrade temperatures to the corresponding absolute 
temperatures, by adding 273°, and record. 

Calculate (a) the increase in absolute temperature ; 
(b) the increase in pressure ; (e) the total pressure of the 
inclosed air at the temperature of steam ; (c?) the decimal 
fraction (three places) which the increase in pressure is of 
the initial (atmospheric) pressure; the fractional increase 
in temperature over the initial temperature, using absolute 
degrees, expressed as a decimal (three places). 

Calculated Results 

0° Centigrade = ° absolute 

° Centigrade (temperature of 

steam} = ° absolute 

Increase in absolute temperature of 

inclosed air ° absolute 

Increase in pressure of inclosed air mm. 

Total pressure of inclosed air at 

steam temperature mm. 

Fractional increase in pressure of air 

Fractional increase in absolute tem- 
perature of air 



LAW OF HEAT EXCHANGE 205 



Discussion : 



Compare the fractional increase in pressure with the 
fractional increase in absolute temperature. How much 
was the increase in pressure for each degree absolute ? 

Conclusion : 

Complete this statement : 

When the volume of a gas is kept constant, the pressure 
of the gas is to its temperature on the scale. 



EXPERIMENT 57 

Law of Heat Exchange 

OBJECT. To find the relation between the heat lost by a hot 
body and the heat gained by a cold body, when the two are brought 
in contact. 

Apparatus. Boiler, with dipper to fit ; calorimeter ; small 
battery jar; perforated cardboard square; graduate (100 cm. 3 ); 
flask (250 cm. 3 ), with 1-hole rubber stopper to fit; 2 thermome- 
ters ; Bunsen burner ; balance ; metric weights ; an ice shaver 
(Fig. 75) is convenient. 

Material. Shaved ice or snow in covered crock ; several 
pailfuls of hot water ; cotton batting. 

Introductory : 

When cream is poured into hot coffee, the mixture be- 
comes cooler than the coffee and warmer than the cream. 
A tub of hot water apparently loses heat when cold water 
is run into it. What really happens is the gaining of heat 
by the cold water at the expeilse of the hot water. Does 
such a transfer of heat take place according to any fixed 
principle ? This question may be answered by mixing 



206 LABORATORY EXERCISES 

weighed amounts of hot and cold water, each of known 
temperature, and taking the temperature of the mixture. 

In order to make the calculations required to establish 
the law of heat exchange, it is necessary to define a unit 
of heat measurement, called the calorie. This is the 
amount of heat which will raise the temperature of one 
gram of water one degree Centigrade. 

Experimental : 

Handle the thermometers carefully, as the glass forming the bulb 
is very thin. Do not pour hot water on a cold thermometer, nor 
cold water on a hot thermometer. Keep your note-book close at 
hand, so as to record the temperatures as soon as read. Read all 
temperatures to tenths of a degree. 

Measure with a graduate 200 cm. 3 of water into the 
dipper of the steam boiler. See that the boiler is about 
half full of water and then light the burner beneath it. 
While waiting for the water to heat, do Part (a). 

(a) Weigh the calorimeter empty and dry. Put 
shaved ice, or snow, into a graduate up to the 15 cm. 3 

mark and then add water 
to the 100 cm. 3 mark. Pour 
the mixture into the cal- 
orimeter and weigh again. 
Keep the outside of the cal- 
orimeter wiped dry during 
Fig. 75. Ice Shaver. . -, . ™ , 

the weighing. Place the 

calorimeter in a battery jar, filling the space between the 
two with cotton wool or other non-conducting packing. 
Cover the top of the calorimeter with a cardboard square, 
having a hole in the center, through which a thermometer 
is inserted. 

Measure 100 cm. 3 of water into an Erlenmeyer flask 
fitted with a 1-hole rubber stopper, carrying a ther- 




LAW OF HEAT EXCHANGE 207 

mometer adjusted so that the bulb is near the bottom of 
the flask when the stopper is in place. 

Warm the water in the flask by dipping the flask into a 
pail of hot water. A slight rotary motion given to the 
flask will insure uniform heating. The water is to be 
heated to as many degrees above the room temperature 
(which will be placed on the blackboard) as the tempera- 
ture of the water in the calorimeter is below the room 
temperature. Read and record promptly the temperatures 
of the two masses of water. 

Then lift off the cardboard cover from the thermometer 
in the calorimeter. Pour the warm water from the flask 
into the calorimeter, letting it run down the thermometer 
which was used in the flask. For about half a minute 
stir the mixture of warm and cold water, using both 
thermometers with the bulbs held together. Read and 
record the average reading of the two thermometers. 

Touch the bulbs of the thermometers to the side of the 
calorimeter to remove any adhering water, and take them 
out of the vessel. Weigh the calorimeter and its contents, 
and record. 

Place one of the thermometers in the water in the calo- 
rimeter and keep it for Part (5), as this is the mass of cold 
water to be used in that part of the experiment. 

(5) By this time the water in the dipper will probably 
be hot. Carefully introduce a thermometer and stir until 
the temperature of the water is ascertained. Record this 
at once. Then quickly read and record the temperature 
of the water in the calorimeter. 

Pour the water from the dipper into the calorimeter, 
and stir with the two thermometers for about half a min- 
ute. Read and record the average reading of the two 
thermometers. Weigh the calorimeter and the mixture. 
Record. 



208 LABORATORY EXERCISES 

Observations 

Part (a) Part (b) 

Weight of calorimeter empty . . g. g. 

Weight of calorimeter and cold 

water g. g. 

Weight of calorimeter and mixture g. g. 

Temperature of cold water . . ° C. ° C. 

Temperature of ivarm (or hot} 

water . / ° C. ° C. 

Temperature of mixture . . . ° C. ° C. 

Temperature of room . . . . ° C. ° C. 

Describe briefly the essential operations in each part of 
the experiment. Make a sectional drawing of the calo- 
rimeter and battery jar, showing how the calorimeter was 
protected from loss or gain of heat from without. 

Calculate in both parts of the experiment the calories 
(1) lost by the warm (or hot) water, (2) gained by the 
cold water. The weights of water mixed can be found 
from the weights recorded. The temperature of the room 
is not to be considered in the calculations. Record the 
results in tabular form at the top of the right-hand page. 

Calculated Results 

Part (a) Part (&) 

Weight of cold water . . . g. g. 

Weight of warm (or ho€) water g. g. 

Fall in temperature of warm 

water ° C. ° O. 

Rise in temperature of cold 

water SO. ° O. 

Calories of heat lost by hot water cal. cal. 

Calories of heat gained by cold 

water cal. cal. 



SPECIFIC HEAT OF A METAL 209 

Discussion : 

Define a calorie. If there was an inequality in the calo- 
ries of heat lost and gained, some heat must have been 
wasted. Could the calorimeter, the thermometers, or the 
air account for heat wasted? Explain. Why is it desir- 
able to have the temperature of the mixture the same as the 
room temperature? Which part of the experiment should 
give you the closer agreement in its results? Why? 

Conclusion : 

Complete the following statement : 

The number of calories of heat lost by a hot body equals 



EXPERIMENT 58 

Specific Heat of a Metal 

OBJECT. To find the specific heat of lead by the method of mix- 
tures. 1 

Apparatus, Lead cylinder with conical top, weighing about 
600 g. to 700 g., having a stout linen thread for suspension; 
spring balance (2000 g.) ; boiler ; Bunsen burner ; ther- 
mometer ; graduate ; calorimeter. 

Introductory : 

An empty tea kettle, placed on the stove, soon reaches a 
temperature equal to that of boiling water. If, however, 
a weight of water be poured into the kettle equal to its 
own weight, it will take several times as long to bring the 
water to the boiling temperature. That is, more heat is 

1 Iron or aluminum may be used in place of lead, if the instructor pre- 
fers ; the lead cylinders, however, can be easily cast, and the use of a 
single solid piece of metal is decidedly preferable to shot. 



210 



LABORATORY EXERCISES 



required to heat one pound or one gram of water one 
degree than is required to heat one pound or one gram of 
iron one degree. No other solid or liquid requires as much 
heat to raise one gram of it one degree as water requires 
for the same change ; the other substances absorb or give 
out less than one calorie per gram per degree. The fraction 
of a calorie absorbed or given out when one gram of a sub- 




Fig. 76. 

stance changes temperature through one degree, is called 
the specific heat of the substance. 

By heating a weighed piece of lead to the temperature 
of boiling water and then cooling it in a known weight of 
water at a certain temperature, the calories given to the 
cold water by the hot lead can be calculated, and also the 
calories yielded by each gram of lead for each degree 
change in its temperature. 

Experimental : 

Fill the boiler half full of cold water and light the 
burner underneath it. Weigh the lead cylinder (Fig. 76, 



SPECIFIC HEAT OF A METAL 211 

A) and record its weight in a table of observations placed 
near the top of the left-hand page. Place the cylinder in 
the boiler, and allow it to remain there for five minutes 
after the water begins to boil freely. 

While the lead is being heated, measure out into the 
calorimeter 300 cm. 3 of cold water from the tap. Record 
the weight of water taken, considering 1 cm. 3 - equivalent 
to a gram. 

When the lead has reached the temperature of the boil- 
ing water, read and record the temperature of the cold 
water. Quickly raise the lead with the thread, touching 
it to the edge of the boiler as it is taken out so as to dis- 
lodge any drops of water, and place the lead in the cold 
water. Stir the water with the thermometer immediately 
after adding the lead, and take the temperature. Record 
this in the table. 

In case you have not determined in a previous experi- 
ment the water equivalent of your calorimeter, obtain its 
value from the instructor. 

Observations 

Weight of lead cylinder g. 

Weight of cold ivater g. 

Water equivalent of calorimeter g. 

Temperature of the lead ° C 

Temperature of cold water ...... ° C. 

Temperature of lead and ivater in calorimeter ° C. 

Make simple outline drawings, showing the three steps 
in the experiment, and describe the method with reference 
to these drawings. 

The weight of the cold water plus the water equivalent 
of the calorimeter, multiplied by their rise in temperature, 
gives the number of calories gained by the cold water and 



212 LABORATORY EXERCISES 

the calorimeter. Record this value in a tabular form near 
the top of the right-hand page. 

This heat gained by the water and the calorimeter was 
given out by the lead in cooling. Assuming the lead to 
be at the temperature of boiling water, compute the deci- 
mal part of a calorie given out when one gram of lead 
cools one degree Centigrade. 

Calculated Results 

Weight of water + water equivalent of calo- 
rimeter ffr 

Temperature change of ivater and calorimeter ° C. 

Total calories gained by water and calorimeter cah 

Total calories given out by lead in cooling. ° C cal. 

Total calories given out by lead in cooling 1° O. cal. 
Calories given out by 1 gram of lead in cooling 

1°0. cal 

Discussion : 

Why is it desirable to have the temperature to which the 
water is raised by the lead, the same as the temperature of 
the room ? 

Conclusion : 

Define specific heat. What do you find the specific 
heat of lead to be ? 



COOLING THROUGH CHANGE OF STATE 213 
EXPERIMENT 59 

Cooling through Change of State 

OBJECT. To observe the heat changes taking place during the 
solidification of acetamid. 1 

Apparatus. Four-inch test tube, three fourths full of acetamid 
crystals, and provided with a one-hole stopper, through which 
passes a thermometer (0° C. to 100° C.) ; ring stand with one 
ring, wire gauze, and clamp for test tube; Bunsen burner ; beaker 
of water. 

Introductory : 

When we melt ice by the use of heat, we notice that it 
takes considerable time. Heat energy must be entering 
the ice, and yet does not warm it. This heat energy 
is used up in melting the ice. In order to freeze water 
back into ice, this heat energy must come out of the 
water. Tubs of water are sometimes placed in cellars to 
prevent vegetables from freezing. As the temperature of 
the cellar falls, the water begins to freeze first. In so 
doing, it gives out heat enough to prevent the air from 
falling as far below the freezing point as it otherwise 
would do. Heat continues to be given out by the water as 
long as it is freezing. 

It is possible to observe these changes more easily in 
some other substances than it is in ice. When we melt 
substances and then allow them to crystallize, they give 
out the same amount of heat which is needed to melt the 
crystals. This heat, which becomes apparent on solidifi- 
cation, makes the substance warm the containing vessel 

1 " Hypo" (sodium thiosulphate) may be substituted for acetamid, but 
the results are not as satisfactory. If hypo is used, the tube, after the 
hypo has been melted, will need to be cooled in a beaker of cold water. 



214 



LABORATORY EXERCISES 



and surrounding objects. We wish to observe the changes 
in temperature before, during, and after the crystallization 
process in some melted acetamid. 

Experimental : 

Support a test tube containing crystals of acetamid 
in a beaker of water on a ring stand (Fig. 77). Melt the 
acetamid by heating the water. As soon as it is com- 
pletely liquefied the thermometer should be inserted in the 
acetamid, so that the bulb shall be entirely covered. If 

necessary, continue to apply heat 
until the temperature is above 
90° C, but not over 95° C. In 
all readings, tenths of a degree 
should be estimated. 

Remove the burner and the 
beaker of water, and allow the 
tube to cool in air, without be- 
ing disturbed in any way. Every 
half minute take a reading of the 
temperature. The tube should 
be closely watched at all times, 
and at the instant solidification 
begins, a reading should be taken 
and marked S in the table, to 
distinguish this point. Continue 
the readings at half-minute inter- 
vals, until solidification is com- 
plete, and then at one-minute 
intervals until a temperature of 
about 55° C. is reached. At the close of the experiment 
the tube and thermometer should be returned to the in- 
structor, without any attempt to remove the thermometer 
from the acetamid. 




Fig. 77. 



COOLING THROUGH CHANGE OF STATE 215 

Record the observations in tabular form near the top of 
the left-hand page. 

Observations 

Time in minutes ^ 1 1 J 2 2|-, etc. 

Temperature in ° C. — — — ■ — — — , etc. 

An outline drawing of the apparatus and a brief descrip- 
tion of the operations should be placed immediately below 
the table of readings. 

Curve. — On a sheet of cross-section paper, plot a curve 
from your readings. Allow two horizontal spaces (2 mm.) 
for a half minute, and one vertical space (1 mm.) for one 
degree. This curve is to be pasted by its edge to the top 
edge of the right-hand page. 

Discussion : 

Answer each question with a complete sentence. 

Is there any point where the temperature curve takes a 
sudden change ? Does this correspond to any change in 
the condition of the acetamid ? Does your curve indicate 
that acetamid has a definite melting (or freezing) point ? 
If so, at what temperature? Is this temperature main- 
tained while solidification is taking place ? Is heat required 
to keep a body at a temperature above that of the room ? 
As no heat is being applied externally, from what change 
in the acetamid must this heat come ? 

Conclusion : 

Does a substance give out heat or absorb heat during 
solidification ? 



216 LABORATORY EXERCISES 

EXPERIMENT 60 

Melting Points and Boiling Points 

OBJECT. To learn the method of determining the melting points 
and boiling points of substances ; and to study the boiling points of 
a mixture of alcohol and water. 

Apparatus. Ring stand ; ring ; two burette clamps ; asbestos 
square, or iron gauze with asbestos center; beaker (100 cm. 3 ) ; 
glass stirrer ; thermometer ; rubber band (section of rubber tub- 
ing ; capillary tubes ; x distilling flask (60 cm. 3 ) ; cork to fit flask 
and perforated to admit thermometer ; small Liebig condenser, or 
2 ft. length of \" tubing, with cork stopper perforated to admit 
delivery tube of distilling flask ; glass beads or a few short pieces 
of glass tubing; small graduate (preferably 25 cm. 3 ) ; Bunsen 
burner. 

Material. Stearic acid ; naphthalene or moth-balls ; carbon 
tetrachloride ; grain alcohol. 

Introductory : 

The melting point of a substance is the transition tem- 
perature between its solid and liquid state. The boiling 
point marks the boundary between the liquid and the 
gaseous states. A considerable change in pressure is 
necessary to affect the melting point of a solid ; the 
temperature at which a liquid boils changes with even the 
ordinary variations of atmospheric pressure. 

Determinations of the melting point are valuable in 
that they indicate the purity of a substance. A pure sub- 

1 The capillary tubes are made by heating the middle of a short piece 
of glass tubing. When the tubing is soft in the heated portion, draw it 
out into a thin-walled tube about 1 mm. in diameter. With a file cut off 
lengths of 2 n to 3" and seal the narrower end of each in the Bunsen 
rlame. 



MELTING POINTS AND BOILING POINTS 217 



stance, melting at a certain definite temperature, melts 
below that temperature when it contains even a very small 
amount of another substance. Crystal- 
line solids are characterized by very 
definite melting points. 

Boiling points are very useful in the 
identification of liquids and as an indi- 
cation of their purity. In the purifica- 
tion or separation of liquids by distilla- 
tion, the observed boiling points are the 
guides to the steps in the process. 

Experimental : 

Melting Points. — (a) Light the 
burner underneath the beaker of water 
(Fig. 78). Have a very small flame, so 
that the water will heat very slowly. 

Put the open end of a capillary tube Fi ~ 78 

into some stearic acid, so as to get a 
column of the solid several millimeters in length. Turn 
the tube upright and tap the closed end gently on the 
table, so that most of the solid falls to the bottom of the 
tube. Slip the tube through the rubber band (Fig. 78, B) 
on the thermometer so that the solid is in 
the position indicated in Fig. 78. 

Move the glass stirrer 1 up and down in 
the beaker until you see some of the small 
particles sticking to the capillary walls melt. 
Read the temperature and record it as the 
melting point of the stearic acid in a tabular form on 
the left-hand page. In case you heated the water too 





Fig. 79. 



1 The bottom of the glass stirrer is most conveniently made by bending 
the glass into a triangular form as shown in Fig. 79. 



218 LABORATORY EXERCISES 

rapidly, let it cool a little and approach the melting point 
more cautiously, using a fresh tube of the stearic acid. 

(5) Determine in a similar manner the melting point of 
naphthalene (the principal constituent of moth balls). 

(<?) Put 15 cm. 3 of carbon tetrachloride into a small 
distilling flask having the delivery tube pointing upward 
as you pour the liquid in. Then arrange the flask as in 
Fig. 70, and pass the delivery tube of flask through a cork 
fitting into a condenser, with a beaker to receive the dis- 
tillate. A few short pieces of glass tube in the flask will 
save time in bringing the liquid to a boil. Take as the 
boiling point of the carbon tetrachloride, the steady tem- 
perature obtained as the liquid distills off through the de- 
livery tube. Record. 

Remove the burner and empty the distilled and the un- 
distilled tetrachloride into the bottle indicated by the 
instructor. 

(d) After rinsing out the distilling flask and the beaker 
with a very little grain alcohol, pour into the flask 15 cm. 3 
of alcohol and 14 cm. 3 of water. This gives a mixture 
which is very nearly 50 per cent alcohol. 

Have at hand a sheet of cross-section paper. Accord- 
ing to a scale given by the instructor, temperatures are to 
be plotted on the vertical axis and the volumes (cm. 3 ) of 
the distillate on the horizontal axis. 

Heat the diluted alcohol to boiling, and plot as the first 
temperature that obtained when the liquid begins to con- 
dense in the delivery tube of the flask. Read the tem- 
perature from this point on as soon as each successive 
3 cm. 3 of the distillate is collected. Plot the readings as 
soon as made. Paste the cross-section paper by an edge 
in the note-book. 



MELTING POINTS AND BOILING POINTS 219 

Observations 

Melting points, Stearic acid ° O. 

Naphthalene °C. 

Boiling point, Carbon tetrachloride .... °C. 

Make drawings showing both the melting-point and 
the boiling-point apparatus. Describe the experimental 
methods with reference to these drawings. 

Discussion : 

The boiling point of ordinary alcohol is 78.4° C. What 
effect does the water in the 50 per cent alcohol have on the 
boiling point of the alcohol ? Between what tempera- 
tures does most of the alcohol distill ? (Examine the 
curve.) How many cubic centimeters of distillate were 
collected between these two temperatures ? What liquid 
is present in the larger amount during the latter part of 
the distillation ? What makes you think so ? Is the boil- 
ing point of water raised when it contains a little alcohol ? 

Conclusion : 

What difference do you notice between the boiling 
point of a pure substance and the boiling point of a solu- 
tion ? How does a liquid dissolved in a second liquid 
affect the boiling point of the second liquid ? 



220 LABORATORY EXERCISES 

EXPERIMENT 61 

Heat Changes during Solution and Evaporation 

OBJECT. To observe the heat changes which accompany solu- 
tion and evaporation. 

Apparatus. Centigrade thermometer ; 50 cm. 3 beaker ; 
wooden block; bicycle pump or foot bellows; two 100 cm. 3 
Erlenmeyer flasks ; battery jar or other receptacle for hypo solu- 
tion ; test tube. 

Material. Strips of cheesecloth one inch wide ; alcohol ; 
ether; " hypo " crystals; supersaturated solution of hypo, made 
by dissolving 100 g. of hypo in 20 cm. 3 of water for each 100 cm. 3 
flask. 

Introductory : 

Photographers notice that a freshly made " hypo " solu- 
tion feels much colder than the water used in making it. 
Is there an actual fall of temperature during solution ? 

Camphor is rubbed on the head for headache ; alcohol 
baths are given to fever patients. On a hot day we feel 
cooler in a breeze. In each of these cases rapid evapora- 
tion takes place on the skin. Is or is not the body actually 
cooled by this evaporation ? 

CAUTION. No flame is to be allowed in the laboratory during 
this experiment, and at the close the windows should be opened wide. 

Experimental : 

(a) A thermometer bulb is wrapped with a strip of 
cheesecloth, which is then tied with a raveling from the 
cloth. The thermometer is held by the upper part of 
the stem and a reading taken. Continue to hold the ther- 
mometer by the stem; then dip the bulb into a test tube 



HEAT CHANGES DURING SOLUTION 221 

of alcohol and remove it when the cloth is thoroughly wet. 
The cloth is allowed to dry, in a draft if possible, the 
temperature being constantly watched. Record the tem- 
perature (1) immediately before dipping into the alcohol, 
(2) immediately after withdrawing the bulb from the 
alcohol, (3) at the reading showing the greatest change 
from the temperature taken in (2). Is the change in 
temperature that you noticed due to the temperature of the 
alcohol, or is it the result of the evaporation of the alcohol ? 

(5) A few drops of water are placed on a wooden block 
and a beaker is set down in the water, so that there will be 
a film of water between the beaker and the block. Enough 
ether is poured in the beaker to cover the bottom. 

Cork the ether bottle tightly and do not inhale the fumes during 
the experiment. 1 

With a bicycle pump or a foot bellows having a piece 
of rubber tubing connected to it, blow gently on the sur- 
face of the ether until it is evaporated. What has hap- 
pened to the water ? If there is no marked change of 
state in the water, repeat, using a little larger amount of 
ether. Has the ether, while evaporating, absorbed heat from 
the water or lost heat to it ? Explain. 

(c) Into a small, clean flask are placed enough crystals 
of hypo to fill the flask a third full. Water, whose tem- 
perature has been observed and recorded, is added till the 
crystals are just covered. The flask is then shaken vigor- 
ously with a rotary motion until as much as possible of 
the hypo has dissolved. The bottom of the flask is then 
felt with the hand. Result ? The thermometer is in- 

1 This part of the experiment must be carried on where there is a good 
draft to remove the ether vapor. If this condition cannot be met, or if 
the class is large, it is advisable to call the class together and perform 
this test as a demonstration. 



/i 



222 LABORATORY EXERCISES 

serted in the solution and the temperature taken and 
recorded. Has the water taken heat from the hypo or given 
heat to it during the process of solution? The result ob- 
tained with hypo is typical of the heat change in solution, 
when no chemical action takes place between the dissolved 
substance and the solvent. 

After the temperature of the solution has been observed, 
it should be placed in a receptacle indicated by the in- 
structor, so that the hypo may be recovered by the evapo- 
ration of the water. 

(c?) At each laboratory table is placed one or more 
flasks with the necks plugged with cotton, each contain- 
ing a supersaturated solution of hypo, which has stood in 
the room long enough to reach room temperature. When 
the students at a table have completed and recorded the 
results of the preceding parts of the experiment, they 
should make this final test together. Each student should 
touch the flask with his finger, without moving the flask 
or disturbing the liquid. The cotton should then be 
removed and a crystal of hypo dropped in. Result ? 
When the change is complete, each student should feel 
of the flask and record his observation. What heat change 
takes place when the hypo is dissolved? When the hypo 
comes out of solution, what heat change occurs ? 

The results of Parts (a) and (<?) should be recorded in 
tabular form near the top of the left-hand page. Other 
observed results should be recorded in the description of 
the part of the experiment to which they belong. 

Observations 
Part (a): 

Temperature of room (1) ° 0. 

Temperature of alcohol (2) . ° C. 

Extreme temperature noticed (3) .... ° C. 



HEAT OF FUSION OF ICE 223 

Part 0) : 

Temperature of ivater before dissolving hypo . ° O 

Temperature of hypo solution ° C. 

Drawings should be made of the apparatus used in 
parts (a) and (6). A brief description of the tests and 
of all results not noted in the table should follow the 
table. 

Discussion : 

Answer, under this heading, the italicized questions 
occurring in the experimental directions. 

Conclusion : 

Is sensible heat absorbed or given out when a liquid 
changes to a gas ? When a solid dissolves ? 



EXPERIMENT 62 

Heat of Fusion of Ice 

OBJECT. To find the number of calories of heat required to 
change one gram of ice to water without warming the ice water 
above the melting point of the ice. 

Apparatus. Calorimeter ; thermometer ; graduate, or balance 
and weights ; fr50 cm. 3 beaker. 

Material. Supply of ice cracked into pieces about the size 
of a hickory nut ; supply of hot water at about 50° C. 

Introductory : 

When water at boiling temperature is thrown upon ice 
that is just ready to melt, some ice will melt and the 
boiling water will be cooled down to the freezing point. 
If just enough boiling water to melt the ice is used, it will 



224 LABORATORY EXERCISES 

be found that there will be one and a quarter times as 
much ice melted as there was boiling water, and the whole 
mass will be ice cold. 

What becomes of the heat that was in the boiling 
water ? When heat is continuously applied to a solid 
body, as when pieces of ice are stirred about quickly in a 
pan on a hot stove, the solid is heated only up to the 
melting temperature. If stirred vigorously, the melted 
part and the part not yet melted do not get warmer than 
the melting temperature until the last bit is melted. 
After this the liquid will get warmer. 

We wish to find how much heat must be applied and 
must disappear as heat energy, when we change a definite 
amount of a solid to its liquid state. This number of 
calories is called the heat of fusion of the substance. 

Experimental : 

(a) In the calorimeter are to be placed 300 cm. 3 of hot 
water. 1 Since the calorimeter is being heated or cooled 
at the same time as the water in it, this fact must be taken 
into account in the calculations. The number of grams 
of water which require the same amount of heat to raise 
them one degree as is required to raise the temperature of 
the calorimeter one degree, will be furnished by the in- 
structor. This number of grams, called the water equiva- 
lent of the calorimeter, is always to be added to the 
number of grams of water actually placed in the calo- 
rimeter. 

(6) Insert the thermometer into the water, and when 
the temperature becomes about 50° C, begin to add dry 

1 If the instructor prefers, the masses of water and ice may be found 
by direct weighing. The method of measurement used here is much 
simpler, and the results are accurate within the limits of error which may 
be expected in the experiment. 



HEAT OF FUSION OF ICE 225 

ice, and continue until enough dry ice to fill a 150 cm. 3 
beaker has been added. Stir constantly. As soon as the 
last particle of ice has been melted, give one final stir and 
take the temperature at once. Record this temperature 
as well as the first temperature, in a table near the top of 
the left-hand page. 

(c) Measure the contents of the calorimeter and record 
the volume obtained. 

Observations 

Volume of hot water cm. 3 

Final volume of ivater and melted ice . . . cm. 3 
Initial temperature (at instant of beginning to 

add ice) °C. 

Final temperature (at melting of last piece of 

ice) °C. 

Water equivalent of the calorimeter ... g. 

Calculation of Results. — (1) Calculate, from the final 
volume of liquid in the calorimeter, the mass of ice used. 

(2) Calculate the number of calories of heat given up by 
the original hot water and the calorimeter, in cooling from 
the initial to the final temperature. This is the total 
number of calories available to melt the ice and warm the 
ice water. Calculate the number of calories used in 
raising the temperature of the melted ice from 0° C. up to 
the final temperature. 

(3) From these two results calculate: 

(a) The total number of calories that were used 

in melting all the ice. 
(5) The number of calories needed to melt one 

gram of ice. 

These calculated results should be entered in a table at 
the top of the right-hand page. 



226 LABORATORY EXERCISES 

Calculated Results 

Total hot mass (mass of water + water equiva- 
lent of calorimeter} g. 

Cold mass (ice) g. 

Change of temperature °C. 

Calories given up by hot water cal. 

Calories absorbed in warming melted ice to 

final temperature . . cal. 

Calories absorbed in melting all the ice . . cal. 

Calories absorbed in melting one gram of ice cal. 

Discussion : 

Explain why it is important to use dry ice. 
Explain how the last three numbers in the table of cal- 
culated results are obtained. 

Conclusion : 

The heat of fusion of ice is calories. 



EXPERIMENT 63 

Heat of Vaporization 

OBJECT. To determine the number of calories of heat that are 
liberated when one gram of steam at ioo°C. is converted into water 
at ioo° C. 

Apparatus. Boiler ; steam trap ; glass and rubber tubing as 
shown in Fig. 80 ; Bunsen burner ; calorimeter ; thermometer ; 
graduate, 100 cm. 3 

Introductory : 

Farmers often cook a large quantity of feed for their 
stock in the following manner : They take steam from 



HEAT OF VAPORIZATION 227 

a boiler through a pipe or hose. The end of this pipe is 
pushed down under cold water in a barrel. The cold 
water condenses the steam and is heated very quickly by 
the heat which the steam gives up. The steam first gives 
up heat in condensing to drops of boiling water, and these 
drops of boiling water give up heat while they cool down 
to the final temperature of the water in the barrel. A 
surprisingly large number of calories of heat is thus given 
to the barrel of water, by a comparatively small weight 
of steam. 

Our experiment is to find out how many calories of heat 
are given out by one gram of steam in condensing to boil- 
ing water, and this number of calories is the same as that 
necessary to vaporize one gram of boiling water, without 
changing the temperature/ This number of calories is 
called the heat of vaporization. 

Experimental : 

The boiler is half filled with water and the burner 
lighted under it. While the water is coming to a boil, 
400 cm. 3 of as cold water as possible are measured into 
the calorimeter. How many grams of water are there ? 
The water equivalent of the calorimeter, or the number of 
grams which must be added to the actual mass of the 
water to allow for the heating of the calorimeter, will be 
given by the instructor, or calculated under his direction. 

In passing the steam from the boiler to the calorimeter, 
errors must be avoided by taking the precautions which 
follow. The steam must be free from water produced by 
condensation. A hot flame and the steam trap included 
in the apparatus, will help to secure this result. The 
temperature of the cold water is to be taken immediately 
before the steam is passed into it. 

The delivery tube should dip far enough below the sur* 



228 



LABORATORY EXERCISES 



face of the water in the calorimeter for the steam to cause 
a rattle as it condenses. At all times the calorimeter 
should be shielded as far as possible from heat other than 
that of the steam passing into it. 

The water should be constantly stirred with the ther- 
mometer, and its temperature watched. When it reaches 
about 40° C, the steam tube should be taken out, the 




Fig. 80. 

water stirred thoroughly, and the highest temperature 
reached after stirring should be recorded. 

You know the number of grams of water with which 
you started. By measuring and recording the contents 
after the steam has passed, the mass of the steam may be 
calculated. 

The observed results are to be placed in a table near the 
top of the left-hand page. 



Observations 



Volume of cold water 
Final volume of water 



cm.* 
cm.* 



HEAT OF VAPORIZATION 229 

Initial temperature of cold water and calo- 
rimeter ° 6 Y . 

Final temperature of calorimeter and contents ° (7. 

Water equivalent of calorimeter g. 

A sectional drawing should be made to show the arrange- 
ment of apparatus and a brief description written, refer- 
ring to the drawing. State the precautions that were taken 
to secure accurate results. 

It is now possible to calculate the number of calories 
absorbed by the cold water, the number given out by the 
condensed steam in cooling, the number given out in con- 
densing, and finally the heat per gram in condensing (heat 
of vaporization). These results should be entered in a 
table placed at the top of the right-hand page, the calcu- 
lations being worked out immediately below. 

Calculated Results 

Total cold mass (mass of water + ivater equiva- 
lent of calorimeter} g. 

Weight of steam condensed g. 

Change in temperature of cold water ... ° C. 

Change in temperature of hot water (condensed 
steam) . ° C 

Calories absorbed by cold ivater in being warmed cal. 

Calories liberated by condensed steam in cooling 
to final temperature ccd- 

Calories liberated by steam in condensing to 
water cal. 

Calories liberated by one gram of steam in con- 
densing cal. 

Discussion : 

What objection would there be in allowing drops of hot 
water condensed in the delivery tube to drop into the calo- 



230 LABORATORY EXERCISES 

rimeter? What is meant by the heat of vaporization of a 
substance ? 

Conclusion: 

The heat of vaporization of water, according to my 
determination, is calories. 



EXPERIMENT 64 

Dew Point 

OBJECT. To find the dew point at the temperature of the labo- 
ratory. 

Apparatus. Bright calorimeter ; thermometer; two beakers ; 
glass stirring rod ; snow, or shaved ice ; fine salt. 

Introductory : 

It has been found by experiment that warm air can 
contain much more water vapor than cold air. When a 
body of warm air saturated with water vapor meets a cur- 
rent of cold air, condensation occurs. Some of the water 
vapor appears as mist, fog, or rain. On a cool night after 
a hot summer day, the ground cools off quickly and chills 
the warm air laden with vapor, so that dew is deposited. 
The temperature to which the air must be cooled in order 
that condensation of water vapor may occur, is known as 
the dew point. This temperature depends upon the relative 
amount of water vapor in the air. 

Experimental : 

Place water to the depth of about an inch in a brightly 
polished calorimeter. In it stand a thermometer and a 
piece of glass tubing to serve as a stirring rod. Place 



DEW POINT 231 

shaved ice or snow in one beaker and fill the other beaker 
with water. 

(a) To the water in the calorimeter, slowly add a little 
ice at a time, stirring thoroughly after each addition. 
Continue until a thin film of moisture appears on the out- 
side of the calorimeter. Note the temperature of the 
water in the calorimeter immediately on the appearance of 
the moisture. Avoid breathing on the calorimeter. Why ? 

A thick deposit of moisture indicates that you have 
cooled the water too rapidly and passed below the dew 
point. In such a case, add small portions of water from 
the other beaker and stir until the mist disappears. 
Then add ice very slowly until the dew point is reached. 

If the air in the laboratory is very dry or quite cool, it 
may be necessary to add a little salt to the crushed ice 
in order to reach the dew point. 

(5) Start with a thin film of moisture on the outside of 
the calorimeter, but have the vessel less than half full of 
the cooled water. Stirring all the time, note the temper- 
ature at which the moisture disappears. This temperature 
should be within a degree of that obtained in (a). 

Observations 

Temperature at which moisture appears. . . °C. 

Temperature at which moisture disappears . . ° (7. 

Make a simple drawing of your apparatus and describe 
the method briefly. 

Take for the dew point the average of the temperatures 
at which the mist appears and disappears. 

Discussion : 

Just what air was cooled to its dew point in this deter- 
mination ? If the air in the room were nearly saturated 



232 LABORATORY EXERCISES 

with water vapor, would the amount of cooling necessary 
to reach the dew point be small or great? Explain, 
How would you find the dew point of the outdoor air on 
a cold day ? 

Conclusion : 

Define the dew point. Complete the following : 

The dew point of the air in the laboratory at on 

was °C. (time) 

(date) 



EXPERIMENT 65 

Magnetic Induction 

OBJECT. To study the behavior of iron, steel, and other materials 
in a magnetic field. 

Apparatus. Strong bar magnet ; pocket compass ; small 
pieces of iron, copper, tin plate, granulated tin, nickel, pasteboard, 
glass; pieces of watch spring; sheets, at least 2 inches square, 
of pasteboard, glass, copper, iron, tin plate ; blocks or other 
supports for magnet and compass ; iron filings or small brads. 

Introductory : 

The most familiar property of a magnet is its ability to 
attract iron and steel. But when two magnets are 
brought near each other, only unlike poles attract, while 
like poles repel. A few simple tests of the behavior of 
iron and steel in a magnetic field will give the principal 
facts of magnetic induction. By this term we mean the 
production of magnetic properties in iron and steel by 
placing these materials in a magnetic field. Other mate- 
rials will also be examined, to determine whether magnetic 
induction takes place in them as well. 



MAGNETIC INDUCTION 233 

Experimental : 

(a) A magnet is successively brought near small pieces 
of iron, steel, copper, "tin" (sheet iron coated with tin), 
granulated tin, nickel, pasteboard, glass. Record in tabu- 
lar form at the top of the left-hand page under the head- 
ing " Magnetic " the names of the materials attracted, and 
under " Non-magnetic," those not attracted. 

(6) A piece of soft iron is held near a magnet, but does 
not touch it. Some iron filings or small brads are then 
brought in contact with the other end of the piece of iron. 
Note and record the result. Without jarring the iron, 
the magnet is then withdrawn carefully and the effect on 
the iron filings noted and recorded. The same test is 
made with a piece of hard steel (watch spring) in place 
of the soft iron, and the results noted. 

(<?) The tests in (5) are repeated with a magnetic 
needle instead of iron filings, and the results noted. 

(d) Unmagnetized pieces of iron and steel are next 
stroked with a magnet in the manner directed by the 
instructor, and then tested with iron filings as in (J), and 
all results noted. 

(0) The magnet and the compass needle are placed on 
convenient supports at such a distance apart as the instruc- 
tor may direct. Sheets of pasteboard, glass, copper, iron, 
"tin" (iron coated with tin), are successively brought 
between the magnet and the needle, and the effect on the 
angle of deflection of the needle noted. 

A brief description of each of the tests made should be 
written on the left-hand page of the note-book immediately 
after making the test, and the results, except in Part (a), 
should be written directly opposite on the right-hand 
page. The following table should be filled out for part 
(a). 



234 LABORATORY EXERCISES 

Observations 

Magnetic Substances Non-Magnetic Substances 



After all the observations have been recorded, the dis- 
cussion should be written on the second right-hand page. 

Discussion : 

Compare the poles produced at the near and at the re- 
mote end of the induced magnet with the inducing pole of 
the permanent magnet. What reason have you for believ- 
ing that there is a pole at the end of the iron near the in- 
ducing pole ? 

What effect does decreasing the distance between the 
iron and the magnet have on the strength of the induced 
poles? (Compare results in (d) with those in (a) and 

Is the reading of a compass needle affected by the brass 
and glass case in which it is mounted ? What material 
would you use to make a shield to protect a watch from 
becoming magnetized ? 

Conclusion : 

(a) Explain, on the basis of the results obtained in this 
experiment, the attraction of a piece of iron or steel by a 
magnet. 

(6) Compare iron and steel with respect to — 

(1) the ease with which they may be magnetized ; 

(2) the permanence of the magnetization. 



MAGNETIC LINES OF FORCE 235 

EXPERIMENT 66 

Magnetic Lines of Force 

OBJECT. To find the direction of the lines of force in certain 
magnetic fields. 

Apparatus. Two 6-inch bar magnets ; 2-inch bar magnet 
(this may be replaced by one of the larger magnets) ; horseshoe 
magnet ; cardboard ; tin pepper box of iron filings, which have 
been heated to thoroughly dry them ; two half-meter sticks or a 
board grooved to hold the magnets. 

Introductory 

If a piece of iron is placed in the neighborhood of a 
magnet, it is subject to a force proceeding from the 
magnet. This magnetic force acts in definite lines, called 
lines of force. -When a magnetic needle is brought into 
the field of a magnet, it always places itself tangent to a 
line of force. Iron filings, when brought into a magnetic 
field and allowed to move freely, become tiny magnetic 
needles and so arrange themselves along lines of force. 
By covering a magnet with a piece of cardboard and 
sifting filings lightly over the cardboard, then tapping 
the cardboard gently, we allow the filings to move freely 
into position along the lines of force in the part of the field 
occupied by the cardboard. 

Experimental : 

The outlines of the magnets, as shown in Figs. 81 and 
82, should be drawn in your book before you come to the 
laboratory, in order that the magnetic field in each case 
may be recorded promptly. 

(a) Lay the bar magnet on the table and place the card- 
board over it, using the half-meter sticks as supports. 



236 



LABORATORY EXERCISES 



W 



C3 K3 



£ZH .O 



Fig. 81. 



First Right-hand 
Page. 



Sprinkle iron filings lightly on the cardboard with the 

sifter held at some distance above the table. Tap the 

cardboard gently to permit the 
filings to arrange themselves. 
When a distinct representation 
of the magnetic field is obtained, 
make an outline drawing of it 
in the upper half of the right- 
hand page of your note-book. 
Be sure that your drawing 
shows the location of the definite 
lines of force along which the 
iron filings arrange themselves. 
Draw a few lines only to show 
the general shape of the field, 

and do not try to represent all the filings. 

(5) Slide the filings from the cardboard on to a sheet of 

paper and return them to the shaker. Arrange the two bar 

magnets with their unlike poles 

facing each other and about 3 

cm. apart. Secure a map of 

the magnetic field on cardboard 

with iron filings as before, and 

sketch in the lower left corner 

of the right-hand page. 

(e) In a similar manner, map 

the field between two like poles 

and record in the other corner of 

the right-hand page. 

QP) Place the small magnet 

at right angles to one end of one 

of the large magnets and about 

3 cm. distant. Map the field as before, and make a draw- 
ing of it in the upper half of the next left-hand page. 





S 
1 




% n\ 


eg 



Fig. 82. Second Left-hand 
Page. 



MAGNETIC LINES OF FORCE 237 

(e) Map the field in the vicinity of the poles of a horse- 
shoe magnet, representing it in the lower half of the second 
left-hand page. 

Write a brief description of the method employed to 
map the fields. No other drawing is necessary. 

Conclusion : 

Do opposite poles seem to be drawn together or pushed 
apart ? What is the effect with like poles ? What special 
advantage is there in the horseshoe-shaped magnet ? 






238 LABORATORY EXERCISES 

EXPERIMENT 67 

Development of an Electrostatic Series 

OBJECT. To arrange various substances in such an order that 
each will be positively electrified when rubbed with the substance 
following it in the series and negatively electrified by the preceding 
substance. 

Apparatus. Gold-leaf electroscope ; l four blocks of wood 
with hard rubber handles, having the following substances ce- 
mented to them : a sheet of glass, a sheet of hard rubber, a 
piece of silk, a piece of cat's fur. 

CAUTION. All the above substances must be thoroughly dry and, 
if possible, warm when they are used. They should be carefully 
tested before the laboratory period, to determine that they are in a 
non-conducting condition. If atmospheric conditions are bad, the 
experiment should not be attempted. 

Introductory : 

The attraction of light objects by rubbed amber was 
the first electrical experiment ever made. The behavior 
of electric charges was investigated quite thoroughly 
before current electricity was produced, and modern 

1 A convenient electroscope is shown in Fig. 83. An 8-oz. wide- 
mouth bottle has a strip of tin foil fastened with shellac across the bottom 
outside, up one side, and through the neck of the bottle down the same 
side within, across the bottom and up the other inside face of the bottle 
to the bottom of the neck. In this way the electroscope can be thor- 
oughly grounded and danger of overcharging avoided. The rod of the 
electroscope is of J" brass, bent into a flat square at the top, as shown in 
Fig. 84 and filed to a double bevel at the bottom. This rod is passed through 
the opening of a one-hole rubber stopper to such a distance that it will 
reach to about the middle of the bottle, and the hole is then filled with 
melted sulphur, to better insulate the rod. A gold leaf, about an inch 
long, is then attached with shellac to each of the beveled surfaces. The 
stopper, with the rod and leaves, is then inserted in the neck of the bottle, 
care being taken not to break the tinfoil on the side of the neck, 



DEVELOPMENT OF AN ELECTROSTATIC SERIES 239 



theories of electricity have a great deal to say about 
electric charges. As a matter of agreement among scien- 
tists, the charge produced on glass, 
when rubbed with silk, is called positive 
( + ); that produced on sealing wax, 
when rubbed with flannel, is negative 
( — ). These are only relative terms. 
In our experiment we shall seek to 
establish a graduated series, with the 
most positive at the top and the most 
negative at the bottom. 




Fig. 83. 



Experimental : 

The electroscope is charged positively 
by induction, using the hard rubber 
plate rubbed with the cat's fur. The 
hard rubber plate after it is negatively 
charged by the cat's fur, is again brought 
down carefully from above to within a 
centimeter of the top of the electro- 
scope and its effect on the divergence 
of the leaves noted. A similar test is 
made with the positively charged fur. The effect of each 
of these charged bodies on the divergence of the leaves 
should be recorded, as these results are the standard with 
which we shall compare the results obtained 
with the other pairs of substances tested. 
In the table record fur as + and hard rubber 
as — . 

The charge on the hard rubber plate is 
then removed by holding the finger at one 
edge and breathing across the surface, or 
by passing the plate quickly through a flame. If there 
is no effect, or only a very slight one, when the plate is 




Fig. 84. 



240 LABORATORY EXERCISES 

again brought near the electroscope, the plate may be con- 
sidered as discharged. Each plate must be similarly dis- 
charged, before being rubbed with a new substance. The 
flame should not be used with the fur. 

After both the glass and the hard rubber have shown 
by test that they have no charge, they are to be rubbed 
together and each in turn brought down carefully from 
above near the top of the electroscope. From the results 
obtained, record this pair in the table, giving each charge 
the proper sign. 

Continue to discharge two substances, then rub them 
together, and finally determine the sign of the charge on 
each, until each of the four substances has been rubbed 
with each of the others. Record all results in the tabular 
form, near the top of the left-hand page. 





Observations 




Charge 


Pairs of substances tested : 




+ 


Oat's fur 


etc 


— 


Hard rubber 


etc 



A careful description of the method of charging the 
electroscope and of the effect on the charged electroscope 
of the fur and hard rubber should be given, accompanied 
by a simple sectional drawing of the electroscope and one 
of the plates under test, with the charge on the" plate and 
on the electroscope marked. 

Place on the right-hand page the following tabular form. 

Summary of Results 

Substance Number of Times Positive Number of Times Negative 



THE SIMPLE CELL 241 

Conclusion : 

From the summary of results, arrange the substances 
in a vertical series, with the one positive the greatest 
number of times at the top, the next most positive next, 
and so on. If you find that your series, as thus arranged, 
fulfills the conditions stated in the Object, place a + sign 
above and a — sign below the column and make a state- 
ment to the effect that this is the correct arrangement of 
the series. 

EXPERIMENT 68 

The Simple Cell 

OBJECT, To study the chemical and electrical action in a simple 
voltaic cell. 

Apparatus. Tumbler of sulphuric acid (1 : 20) ; strips of amal- 
gamated and unamalgamated zinc ; strip of copper ; galvanometer 
or low-reading voltmeter ; l battery stand or clamps for holding 
elements in place ; No. 18 insulated copper wire for connections. 

Introductory : 

When two conductors are placed in a solution which 
acts on one of them more than on the other, a difference 

1 It is the belief of the authors that voltmeters and ammeters are to be 
preferred to galvanometers, as they introduce the student directly to 
practical units. High-grade commercial instruments of the d'Arsonval 
type may now be had at prices which make the original investment but 
little more than that for galvanometers, while the trouble and expense of 
keeping galvanometers in order is far more than for the commercial in- 
struments. Ammeters should have external shunts ; the instrument 
movement without the shunt may then be used as a galvanometer in 
Wheatstone bridge and induction experiments. The scales recommended 
for the ammeters are 12 amperes and 1.2 amperes. The voltmeters should 
have 120 volt and 6 volt scales. Where voltmeters are not available, 
d'Arsonval galvanometers may be used in many experiments. Tangent 
galvanometers, or shunted d'Arsonval instruments, may be substituted 
for ammeters. 



242 



LABORATORY EXERCISES 



of potential is produced between the two conductors. 
When they are joined by a wire, an electric current flows 
from one to the other. A strip of zinc and a strip of 
copper immersed in dilute sulphuric acid constitute a 
simple cell. We wish to investigate the chemical and 
electrical action that takes place in such a cell. 

Experimental : 

A strip of zinc and one of copper, a tumbler con- 
taining dilute sulphuric acid, clamps for holding the 
strips in place, connecting wires, and a voltmeter will be 
furnished you. The chemical action on the strips is 
tested by placing each in the acid separately, then both 
together, first unconnected and then connected (Observa- 
tions 1-4). The relative number of bubbles produced at 
each plate should be noted and recorded in Observations 
1-5. 

The order of operations is indicated in the table of 
observations, to which the numbers in the text refer. 
Where there is not room to write the 
observed result on the left-hand page, 
it may be continued on the same line 
of the right-hand page. 

Amalgamated zinc (zinc coated with 
mercury) is next substituted for the 
plain zinc and connected by a wire with 
the copper (Observation 5). A wire 
from each plate is separately touched to 
the tongue (Observation 6), and then 
both wires are touched to the tongue 
at different points (Observation 7). 

The wires are connected to a voltmeter. When the 
needle swings over the scale in a positive direction, the 
current leaves the cell by the wire connected to the plus 




Fig. 85. 



THE SIMPLE CELL 243 

terminal of the voltmeter. The terminal of the cell to 
which this wire is connected is the plus electrode, or 
cathode. Read the deflection if the needle is on the scale. 
Reverse the connections at the voltmeter and determine 
whether the current has a definite direction (Observations 
8, 9, 10). 

Observations Results 

1. Copper in acid 

2. Zinc in acid 

3. Both in acid, unconnected . 

4. Both in acid, connected 

5. Zinc amalgamated, con- 

nected to copper 

6. Each wire touched to 

tongue separately 

7. Both wires touched to 

tongue 

8. Copper connected to volt- 

meter + , reading .... 

9. Zinc connected to volt- 

N meter + , reading 

10. CathodeQ + } plate of cell 

is 

Make a drawing of the stand, the tumbler, and the 
plates, in the usual place on the left-hand page, and in- 
clude in your description any points not noted in the 
table. 

Explanation of the Chemical Action. — (Not to be 
written in the note-book.) The production of gas in the 
liquid shows that chemical action is going on. The gas 
is hydrogen, and results from the decomposition of the 
acid. The remainder of the acid unites with the zinc, 



244 LABORATORY EXERCISES 

forming a soluble compound. The action of the acid on 
the zinc, when the plates are not connected, is called 
local action. The deflection of the voltmeter corresponds 
to the electric pressure. A loss of electric pressure 
after the cell has been in action for some time is due to 
polarization. 

Discussion : 

How is local action prevented or diminished ? Why is 
it desirable to prevent it? Give two ways of showing 
the passage of a small current through a wire. Which 
test might furnish a method of determining the direction 
of the current flow ? How ? 

Conclusion : 

State the essential parts of a simple cell. 



EXPERIMENT 69 

The Two-fluid Cell 

OBJECT. To study the prevention of polarization in the Dan- 
iell cell. 

Apparatus. Tumbler ; porous cup ; battery stand ; amalga- 
mated zinc ; copper strip ; voltmeter or high resistance galva- 
nometer ; resistance box, or coil of wire having a resistance of 
about 20 ohms ; $ 18 insulated copper wire. 

Material. Dilute sulphuric acid (1 : 20) ; saturated solution 
of copper sulphate. 

Introductory : 

When the push button of a doorbell is pressed for a long 
time, the bell will often stop ringing. A change has taken 
place in the battery which prevents a current sufficient 



THE TWO-FLUID CELL 



245 



to ring the bell from passing. This change is an increase 
in resistance and a decrease in the difference in potential 
between the plates of the cell ; it is called polarization. 
We are going to observe the polarization of a simple cell, 
and see how it is prevented in a two-fluid cell, called the 
Daniell cell. 



Experimental : 

(a) A simple cell is set up as in Experiment 68. The 
difference of potential of the freshly prepared cell is read 
by means of a voltmeter. The cell is then allowed to 
send a current through a coil of wire connected to its 
terminals, and the voltage is read immediately after con- 
necting the coil. The difference between this and the 
first reading is due to the fact that only the part of the 
pressure which is driving the current through the coil is 
now being measured. The voltmeter is carefully watched 
until the needle becomes stationary, 
when a reading is taken. Any differ- 
ence between the second and third 
reading is due to polarization. 

Notice whether there are hydrogen 
bubbles on the copper plate. Record 
result. If bubbles are noticed, rub 
them off with the finger, and again 
observe the voltmeter reading. 

(b) Part of the acid is poured into 
a small porous cup. This is set into 
the tumbler containing the remainder 
of the acid. The plates are then inserted, the zinc into the 
acid in the porous cup and the copper into the acid in the 
tumbler. The voltage is read before connecting the coil, 
immediately after connecting the coil, and when the 
needle becomes stationary, as in (a). Does the porous cup 




Fig. 86. 



246 LABORATORY EXERCISES 

prevent polarization? Notice whether bubbles form, as in 

(«)• 

(e) Keeping sulphuric acid in the porous cup around 
the zinc, replace the acid in the tumbler with copper sul- 
phate solution. The cell now has zinc in sulphuric acid 
and copper in copper sulphate, and is known as a Daniell 
cell (Fig. 86). Make three readings of voltage, under the 
same conditions as in the preceding parts of the experi- 
ment. Does the copper sulphate prevent polarization ? 

Record the results of your observations in tabular form 
near the top of the left-hand page. 

Observations 

Part (a) Part (b) Part (c) 

Bubbles appear at . 

Voltage before closing circuit 

Voltage, circuit just closed 
Voltage, needle stationary. 

A diagrammatic sketch, similar to Fig. 86, should be 
made for each of the three tests. In each sketch, label each 
plate and the contents of the tumbler and the porous cup. 
A very brief description should accompany these drawings. 

Discussion : 

How does the collection of hydrogen on the copper 
plate affect the voltage of the cell ? Which is more prac- 
tical, the simple cell or the Daniell cell ? Why ? 

Answer also the questions in italics occurring in the ex- 
perimental directions. 

Conclusion: 

What prevents polarization in the Daniell cell ? 



ELECTROPLATING 247 

EXPERIMENT 70 

Electroplating 
OBJECT. To electroplate (a) with copper ; (b) with nickel. 

Apparatus. Porcelain battery top, or Skidmore battery 
stand; electric light carbon ; copper sheet 4"x2", with wire at- 
tached; strip of pure nickel about 3"x 1" ; two storage cells, or 
three Daniell cells ; two tumblers ; wire for connections ; reversing 
switch (Fig. 88) is desirable. 

Material. Saturated solution of copper sulphate ; plating 
bath of nickel ammonium sulphate ; l rouge cloth or other polishing 
material. 

Introductory ; 

The simplest and most convenient method of plating an 
object is by means of the electric current. The positively 
charged metallic ions travel with the current and deposit 
at one electrode. The object to be plated should be an 
electrode in a solution of some compound of the metal to 
be deposited. If a current of suitable strength is then 
passed, a film of the metal coats the object. To supply 
the place in the solution of the metallic ions deposited, a 
strip or bar of the plating metal is hung in the solution 
and serves as the other electrode. If the object to be 
plated is an insulator, it must first be coated with some 
conducting material, such as graphite. An object to be 
nickel plated is usually copper plated first ; the nickel is 
then plated on the copper coating. 

1 This solution is made by dissolving in one liter of water, 72 g. of 
nickel ammonium sulphate, 23 g. of ammonium sulphate, and 5 g. of 
crystallized citric acid. Then ammonium hydroxide is added until the 
solution is no more than slightly acid to blue litmus. If, after some time, 
the solution does not plate well, more ammonium sulphate should be 
added. The bath should always have a slightly acid reaction. 



248 



LABORATORY EXERCISES 



Experimental : 

(#) Copper Plating. — Fasten an electric light carbon 
in one clamp, and the wire attached to a copper strip in 

the other clamp of the stand 
or battery top furnished you. 
The copper should be bent 
into cylindrical form, en- 
circling the carbon, but not 
touching it at any point 
(Fig. 87). Immerse the 
carbon and the copper in 
a tumbler of copper sul- 
phate solution. Is there 
any action ? 

Connect the copper termi- 
nal with the positive termi- 
nal of two storage cells (or 
three Daniell cells), connected in series. Allow the cur- 
rent to pass for five minutes. Withdraw both the carbon 
and the copper and examine them. 

Replace them in the solution, reverse the direction of 
the current through the plating cell, and leave them for 
five minutes. Again examine. 
Decide upon the correct 
connection for plating the 
carbon and allow the cur- 





Fig. 



To Source 
of Current 

Reversing Switch. 



rent to pass long enough to 
form a firm deposit. When 
the carbon is well coated, 
take it out of the solution 
and allow it to dry; then polish it with rouge cloth. 
Which arrangement of the carbon and copper is correct ? 
Why ? Upon which terminal, anode or cathode, is the metal 



ELECTROPLATING 249 

deposited? Where did the deposited metal come from? 
What is the use of the copper strip ? 

(ft) Nickel Plating- — Wash all the copper sulphate 
solution from the clamps for holding the electrodes. 
Fasten the copper-plated carbon and a strip of nickel in 
the two clamps. Immerse the electrodes in a tumbler of 
nickel ammonium sulphate solution. Pass the current, 
making the nickel the anode, for five minutes, noting the 
action. The pressure should be about 2.2 volts. When 
a good coating of nickel is obtained, remove the cathode 
from the solution and examine it. Compare the thickness 
of the coating on the side near the nickel electrode with 
the coating on the other side. When dry, polish with 
rouge cloth. 

Would it be better if the nickel anode surrounded the car- 
bon, as the copper anode did? 

Make a simple diagram, showing the parts of the plating 
cell, and the direction of the current when plating occurs. 
Also indicate the source of current. With reference to 
the diagram, describe the operations in (a) and (5), giving 
the results in each case. 

Discussion : 

Under this heading on the right-hand page, answer the 
questions occurring in the experimental directions. 

Conclusion : 

Complete the following statement : 

In electroplating, the object to be plated is the , the 

plating metal is the , the solution furnishes _._. 



250 LABORATORY EXERCISES 

EXPERIMENT 71 

Electrotyping 
OBJECT. To make a small electrotype. 

Apparatus. Skidmore stand or porcelain battery top ; copper 
strip 1 " x 5" ; lead strip 1" x 5" ; tumbler ; beaker ; three Dan- 
iell cells ; x wire for connections ; small brush ; Bunsen burner ; 
pieces of type or seals. 

Material. Powdered graphite ; beeswax ; saturated solution 
of copper sulphate ; 5 per cent solution of zinc sulphate ; pieces of 
cloth. 

Introductory : 

A printer in his smaller job work prints the copies from 
the type set up by the compositor. When the number 
of copies desired runs up into the thousands, as in a large 
edition of a book, the type metal is not hard and durable 
enough to give such a large number of clean-cut impres- 
sions. Accordingly a wax impression is made of the type 
as set. The wax impression, covered with a conducting 
material, as graphite, is then electroplated with copper. 
The thin coating of copper, which has taken the form of 
the wax mold, is stripped from the wax, backed with 
some easily fusible metal, and mounted on a wooden 
- block. In this way an electrotype is made with a hard 
surface of copper in the form of the original type. 

Experimental : 

Hold the strip horizontally above a small Bunsen flame, 
so that some pieces of beeswax, placed on the upper sur- 

1 As the Paniell cells are run over night, the zinc plates should be well 
amalgamated and a 5 per cent solution of zinc sulphate be used instead 
of sulphuric acid. When not in use, short-circuit these cells. 



ELECTROTYPING 



251 



face of the strip, will melt and cover uniformly two thirds 
of the strip with a coating about | inch thick (Fig. 80). 

When the wax has cooled and hardened, rub. with a 
cloth finely powdered graphite over the wax and beyond 
it to the surface of the lead, in order to prepare a conduct- 
ing surface. Enough graphite should be used to make a 
firm, shiny coating. 

Take the type or other object to be copied and rub 
graphite over its surface. Then 
press the type into the wax, 
until a clean-cut impression 
extends nearly but not quite 
through the wax, when the type 
is removed. Dust the impres- 
sion again with graphite, tak- 
ing care not to mar the outline. 

With a brush and melted 
beeswax, coat the back and 
edges of the lead strip up to the 
point where it is to be clamped. 

Clamp the lead and copper 
strips in place, so that the 
impression is toward the copper strip. Immerse the 
strips in a tumbler of copper sulphate solution. The 
electrodes should be about one centimeter apart. Ar- 
range three Daniell cells in series and connect them with 
the electrodes in the plating solution, making the copper 
the anode. After the current has been running for five 
minutes, remove and examine the lead strip. Coat with 
melted wax any place where copper has been deposited 
outside of the impression which you wish to copy. 

Return the electrode to the solution and allow the cur- 
rent to pass until the laboratory period next day. 

At the next laboratory period, remove and wash off the 




Fig. 89. 



252 LABORATORY EXERCISES 

lead strip. Immerse it in hot water so as to soften the 
wax, and then with the aid of a knife strip off the depos- 
ited copper carefully in one piece. The last pieces of 
adhering wax may be removed by heating the copper and 
wiping it with a cloth. 

Back the copper with melted tin to the thickness of | 
of an inch, in case the instructor gives directions for so 
doing. Otherwise put the electrotype in an envelope and 
attach the envelope to the note-book page by the flap. 

Make a diagram showing the arrangement of the appa- 
ratus and a drawing showing the lead strip with its coat- 
ing and impression. Describe the experimental method 
with references to these drawings. 

Discussion : 

What was the use of the copper strip ? Of the lead 
strip ? Why was the current allowed to run all night ? 



EXPERIMENT 72 

The Storage Cell 

OBJECT. To study the construction and action of a simple stor- 
age cell. 

Apparatus. Tumbler; two lead plates, about 3" x 1"; Skid- 
more or other battery clamp ; voltmeter or galvanometer ; elec- 
tric bell ; # 18 insulated copper wire for connections. 

Material. Sulphuric acid (1 of acid to 8 of water) ; sand- 
paper. 

Introductory : 

The essential conditions for the production of a voltaic 
cell are two different plates and a solution that will react 



THE STORAGE CELL 253 

chemically with one of them more than with the other. 
The limit to the usefulness of such a cell is reached when 
one of the plates or the electrolyte is used up. This 
fact makes the primary cell a very expensive source 
of current. In the lead secondary or storage cell, this 
difficulty is avoided. The two plates of this cell are alike 
before " charging." The cell is charged by passing a cur- 
rent through the plates and the electrolyte. The latter is 
decomposed and the products of decomposition add oxygen 
to one plate and take oxygen from the other, thus making 
the plates different chemically. When the cell is used as 
a source of current, a reverse action takes place — the 
plates again becoming alike and the electrolyte being 
restored to its original form. This process can be re- 
peated a great many times before it is necessary to put 
in new plates. 

Experimental : 

(a) The lead plates are to be thoroughly cleaned with 
sandpaper until the surface is bright. Then set the 
plates in a tumbler of dilute sulphuric acid and clamp 
them so they will not make electrical 
contact with each other. Connect 
the plates to a voltmeter or galva- 
nometer. Is there any difference of — o < — 

potential between the lead plates when " + ° 
immersed in sulphuric acid ? 

(5) Without disconnecting the 
voltmeter, connect the two plates with Fi 90> Cell charging. 
a source of current having a pressure 
of about 4 volts. Reverse the connections of the meter, if 
necessary, so that the needle remains on the scale. Note 
the reading of the meter and record. Observe also 
whether bubbles collect at either or both plates. If at 




254 



LABORATORY EXERCISES 



both plates, at which are they produced more freely, anode 
or cathode ? Pass the current for two minutes, then dis- 
connect the source of current. Observe and record any 
deflection of the meter when the current is no longer pass- 
ing into the cell from an outside source. Short-circuit the 
cell, by connecting the plates with a wire, for a minute or 
so. Disconnect the short-circuiting wire and again read 
the meter. 

(<?) Again charge the cell, this time for from 5 to 10 
minutes. At the end of the charge take the meter reading, 
as before. Is the plate which is the anode when charging, 
the anode or cathode when discharging ? 

Remove the plates and observe any change in appear- 
ance that has taken place. Replace the plates and con- 
nect them to an electric bell. Result ? By short-circuit- 
ing the cell, bring it back to an uncharged condition, as 
shown by the meter. 

Most of the observations made can be recorded by fill- 
ing in the proper spaces in the following tabular form to 
be placed near the top of the left-hand page. 



Observations 





Before 
Charging 


While 
Charging 


Fully 
Charged 


Discharged 


Voltmeter reading. . . 
Bubbles at anode . . . 










Bubbles at cathode . . 








Color of anode .... 








Color of cathode . . . 













Note. In the table fill in spaces marked ( ), but leave blank 

spaces marked ( ). 






LAWS OF RESISTANCE 255 

Make a diagrammatic sketch showing the connections of 
the apparatus and write *a brief statement of the steps of 
the experiment. Include in the description any observed 
facts not already noted in the table. 

Discussion : 

Lead peroxide is chocolate colored. Which plate, the 
lead or the lead peroxide, is the positive plate when the 
cell is charged ? Does the cell store electricity or chemical 
energy which can be converted into electricity ? 

Conclusion : 

State the action which takes place in charging and in 
discharging a storage cell. 



EXPERIMENT 73 

Laws of Resistance 

OBJECT. To determine how the resistance of a wire is related 
to its length, area of cross section, and material. 

Apparatus. Resistance board, on which are stretched the 
following wires, connected in series : 2 meters # 28 copper ; 2 
meters # 28 copper ; 2 meters # 22 copper ; 4 meters # 28 
iron ; battery furnishing about 6 volts ; low-range voltmeter and 
ammeter, or d'Arsonval and tangent galvanometers; # 18 insu- 
lated wire for connections. 

Introductory : 

There is a difference in the filaments of a 16 candle 
power lamp and a 32 candle power lamp, made for use at 
the same voltage. The filament of the more powerful 
light allows about twice as much current to pass as the 16 



256 



LABORATORY EXERCISES 



candle power filament does. This difference in current is 
due to a difference in the resistance of the two filaments. 
The difference in resistance is secured by making the fila- 
ment of different dimensions. With the same number of 
volts applied, a copper wire will permit a greater current 
to pass than a German silver wire of the same dimensions. 
Here it is the material that makes the difference in resist- 
ance. By experimenting with wires of known lengths, 
areas, and materials, the effect of each of these on the re- 
sistance of the wire may be determined. 

Experimental : 

The wires to be tested are mounted on a board, provided 
with binding posts at the end of each wire. By connecting 
a battery to the two outside binding posts, a current is 




Fig. 91. A, 2 meters copper #28 ; B, 2 meters copper #28 ; C, 2 meters 
copper #22; D, 4 meters iron #28. 

sent through the wires in series. In order to read the 
value of the current, an ammeter is inserted between the 
battery and the resistance board. A voltmeter is provided 
with wires which can be connected to any pair of binding 



LAWS OF RESISTANCE 



257 



posts (Fig. 91). The length in meters of each wire and 
its area in circular mils will be marked on the board or 
may be measured. A circular mil is a circle whose 
diameter is one one-thousandth of an inch. The area of 
ft 28 wire is approximately 160 circular mils and the area 
of ft 22 wire is approximately 640 circular mils. 

After the connections have been made as just described, 
the current through the wire and the drop of potential 
between the ends are read and recorded in each of the 
following instances : 

(a) 2 meters of ft 28 copper wire. 
(5) 4 meters of ft 28 copper wire, 
(e) 2 meters of ft 22 copper wire, 
(d) 4 meters of ft 28 iron wire. 

Observations 



Teial 


Length of 
Wire 


Area of 
Wire 


Mateeial 


Current 


Pressure 


a 


HI. 


cm. 


copper 


amp. 


Y. 


b 


m. 


cm. 


copper 


amp. 


V. 


c 


m. 


cm. 


copper 


amp. 


V. 


d 


m. 


cm. 


iron 


amp. 


V. 



A diagram should be made showing the connections of 
the apparatus, and a brief description of the method of the 
experiment should follow the table of observations. 

By comparing the results of (a) and (5) the effect of 
length on resistance may be obtained ; (<z) and (<?) will 
show the effect of area of cross section. The resistances 
obtained in (6) and (d) will show the comparative resist- 
ances of copper and iron. The resistances may be calcu- 
lated by the application of Ohm's Law. 



258 LABORATORY EXERCISES 

Calculated Results 

Trial abed 

Kind of wire 

Resistance 

Discussion : 

Explain the method of calculating the resistance of the 
wires. 

Conclusion : 

State the relation between the resistance of a conductor 
and its length ; the relation between the resistance and 
the area of cross section. 

How many times is the resistance of iron as great as 
that of copper ? 

EXPERIMENT 74 

Effect of Temperature on Resistance 

OBJECT. To observe the change in resistance of various con- 
ductors with a change in temperature. 

Apparatus. Coil of iron wire, wound on a porcelain insulat- 
ing tube ; x similar coils of German silver wire and of some wire of 
very low temperature coefficient, such as manganin, or " la la" ; 
ammeter, or low resistance galvanometer ; iron tripod for support- 
ing coils ; Bunsen burner with' wing top ; wire for connections ; 
3 storage cells. 

1 The porcelain insulating tubes can be obtained from any dealer in 
electrical supplies ; binding posts are mounted on the ends of wooden 
plugs inserted in the ends of the tube, and the wire, wound tightly 
around the porcelain, is clamped between the binding post and the wood. 
(Fig. 92.) la la wire can be purchased of H. Boker and Co., 101 Duane 
St., N.Y. ; manganin wire is sold by the Central Scientific Co., Chicago. 



EFFECT OF TEMPERATURE ON RESISTANCE 259 



an 



■— 



Fig. 92. Coil wound on Tube. 






Introductory : 

The temperature of the conductors in the field and in 
the armature of a dynamo is higher than that of the sur- 
rounding air when the ma- 
chine is running at full load. 
Will they have the same 
resistance as at ordinary 
temperatures ? Can we, by 
measuring the resistance of a cold incandescent lamp, 
determine how much current the lamp would take at the 
voltage necessary to make the lamp glow brightly ? Do 
all conductors behave alike with regard to the effect of 
temperature on their, resistance ? These are some of the 
questions which this experiment is designed to answer. 

Experimental : 

(a) Support the coil of iron wire on a tripod, in such 
a way that a considerable part can be heated. Arrange a 
circuit having the coil of iron wire, the battery and the 
ammeter in series. Observe and record the reading of the 
ammeter. Place the lighted burner under the central part 

of the coil and take another 
reading of the ammeter when 
the coil becomes red-hot 
(Fig. 93). 

(5) Using the same source 
of current, read the current 
through the coil of German 
silver wire, cold and hot. 
(<?) Take the same read- 
ings with the coil of special resistance wire, the name of 
which will be given you by the instructor. 1 

1 If it is desired to extend the experiment to carbon, the resistance of 
an incandescent lamp can be found when cold, by means of a Wheatstone 




260 



LABORATORY EXERCISES 



Observations 



Trial 


Material 


Temperature 
(Hot or Cold) 


Current 


a 
a 
b 
b 
c 
c 


Iron 

Iron 

German silver 

German silver 


Cold 

Hot 

Cold 

Hot 

Cold 

Hot 


amp. 

amp. 

amp. 

amp. 

amp. 

amp. 



Make a simple drawing, showing one of the coils being 
heated, with the ammeter and battery connected in circuit. 
A brief description of the experiment should accompany 
the drawing. 

Record in tabular form, at the top of the right-hand 
page, whether the resistance of each material is increased 
or decreased by an increase of temperature. Remember 
that, with the same voltage applied, an increase in current 
means a decrease in resistance. 



Deductions 

An increase of temperature the resistance of iron. 

An increase of temperature the resistance of Ger- 
man silver. 
An increase of temperature the resistance of 

Discussion : 

Metals in general behave like iron. 

Account for the fact that a tungsten lamp takes sev- 
eral times as much current at the instant when the current 
is turned on as it does a few seconds later. Why is the 

bridge ; and then at normal voltage by the voltmeter and ammeter method. 
If this is done, the results should be recorded in a separate table. 



INTERNAL RESISTANCE OF A CELL 261 

special resistance wire which you have tested better for use 
in a resistance box than German silver ? 

Conclusion : 

What is the effect of an increase in temperature on 
the resistance of most metals? 



EXPERIMENT 75 

Internal Resistance of a Cell 

OBJECT. To determine the effect of the size of the plates and 
the distance between them on the internal resistance of a cell. 

Apparatus. Tumbler ; porous cup ; amalgamated zinc ; cop- 
per plate ; porcelain top for holding plates so that their distance 
can be varied ; voltmeter; ammeter; $ 18 insulated copper wire 
for connections. 

Material. Dilute sulphuric acid (1:20); copper sulphate 
solution. 

Introductory : 

Dry cells and other cells are made in different sizes. 
It is natural to suppose that there is some difference in 
the performance of one of the three tiny cells contained 
in a pocket flash lamp, and one of the large dry cells used 
for ignition in an automobile. By using a cell in which 
the area of the plates immersed and the distance between 
them can be varied, we can determine what effect the size 
and distance of the plates has on the voltage and on the 
amperage of the cell. 

Experimental : 

From the materials furnished you, assemble a Daniell 
cell. The distance between the plates of your cell may be 



262 



LABORATORY EXERCISES 



varied by moving the clamps which hold the plates toward 
or away from each other. This will change the length of 
the liquid conductor by which the current flows through 
the cell, without changing its cross section. The cross 
section of the liquid conductor depends upon the area of 
the plates immersed in the electrolytes, and may be 
changed without varying the length. The materials of 
the conductor remain unchanged throughout. 

Except when taking readings, keep the circuit open. 
The terminals of the cell should be connected to one in- 
strument only at a time, and not to both. 

(1) Immerse the plates as far as possible, and bring 
them as near together as the walls of the porous cup will 
permit. Read the voltmeter and ammeter separately and 
record in the table of observations. 

(2) Separate the plates as far as the walls of the tum- 
bler will permit. Take and record the reading of each 
instrument. 

(3) Keeping the plates at the same distance as in (2), 
raise them until the plates project only 1 cm. into the 
liquids. Read and record as before. 

Observations 



Trial 



Position op Plates 



Close 

Separated 

Separated 



Length of Plates 
Immersed 



Entire 
Entire 
1 cm. 



Yolts 



Amperes 



Make simple sectional drawings of the cell, showing the 
position of the plates and the amount immersed for each 
case. A very brief description should accompany these 
drawings. 



GROUPING OF CELLS 263 

Discussion : 

Does the electromotive force of the cell depend upon 
the materials or upon the length of the liquid conductor ? 
Upon what conditions does the current furnished de- 
pend ? Will a large Daniell cell have a higher electro- 
motive force than a small one ? Will it furnish more 
current ? 

Conclusion : 

Applying Ohm's Law, state how the resistance of a cell 
is affected by the size of the plates and by the distance 
between them. 



EXPERIMENT 76 

Grouping of Cells 

OBJECT. To determine the proper connection of two cells to 
secure the greatest current, (a) when the external resistance is low ; 
(b) when the external resistance is high. 

Apparatus. Two student's Daniell cells, tumbler form ; resist- 
ance box ; connection board, with switches and connections as 
shown in Fig. 94 (double connectors may be substituted for 
switches if necessary) ; ammeter. 

Introductory : 

If two like pumps are placed side by side, drawing water 
from the same reservoir and delivering into the same 
pipe, the two pumps will deliver twice as much water as 
one pump can deliver, but at the same pressure. These 
pumps may be said to be in parallel. If, however, the 
two pumps were so placed that the second took its water 
from a pipe to which it had been delivered by the first, 
the amount of water delivered would be no greater than 



264 



LABORATORY EXERCISES 



that delivered by one pump, but the pressure of the water 
would be twice as great. These pumps may be spoken of 
as in series. 

Voltaic cells may be arranged either in parallel or in 
series. The arrangement which will yield the greater 
current depends upon the external resistance, as compared 
with the combined resistance of the cells. By using a 
low external resistance and a high external resistance, 
with the cells connected in each of the two ways, a gen- 
eral conclusion may be reached. 

Experimental : 

(a) Two small-sized Daniell cells are set up. By the 
use of a combination of switches, as shown in Fig. 94, or 
by the use of simple connecting wires, the zinc of one cell 

is connected to the copper of the 
other. The resistance box and 
the ammeter are connected in 
series with the two remaining 
terminals. After making the 
connections, inspect them to see 
that all the current must pass 
through each part of the circuit ; 
this is the test of a series con- 
nection. Withdraw the 0.2-ohm 
plug from the resistance box. 

V- 4 — J V J Read the ammeter and record in 

\ / \ J ^e t a kle of observations placed 

near the top of the left-hand page. 
Replace the 0.2-ohm plug, and, 
without changing connections, remove the 20-ohm plug. 
Read the ammeter and record. 

(J) Connect the two copper plates and connect the two 
zinc plates. To the combined copper terminal connect 




Fig. 94. 



GROUPING OF CELLS 265 

the + terminal of the ammeter, and then connect the re- 
sistance box between the other terminal of the ammeter 
and the combined zinc terminal of the cells. Be sure that 
the coppers of the cells have no other connection with the 
zincs, except through the ammeter and resistance box. 
The cells are now connected in parallel with each other 
and are sending a current through the resistance box, and 
the same current through the ammeter. Take readings 
through the 0.2-ohm coil and through the 20-ohm coil, 
and record, as in (a). 

Observations 

Connection of Cells Series Series Parallel Parallel 



Resistance ._. 


ohms 


ohms 


ohms 


ohms 


Current 


amp. ._ 


amp. 


amp. 


amp. 



Make two connection diagrams, one showing the cells 
in series connected with the resistance box and ammeter, 
and the other showing the cells in parallel connected with 
the resistance box and ammeter. A brief description 
should accompany the diagrams. 

Discussion : 

As the resistance of electrical apparatus is in general 
much higher than the battery furnishing the current 
would have in either arrangement, which will be the 
usual method of connecting voltaic cells ? 

Conclusion : 

With what kind of external resistance do cells in par- 
allel furnish the greater current ? With what kind of 
external resistance are cells in series better ? 



266 LABORATORY EXERCISES 



EXPERIMENT 77 

Resistance and Current in a Divided Circuit 

OBJECT. To compare, (a) the currents in the branches of a 
divided circuit with the resistance of those branches ; (b) the total 
resistance with the resistance of the branches. 

Apparatus. Lamp board like that shown in Fig. 95 l ; 32 
candle power lamps to fill board; 3 ammeters; voltmeter, with 
connecting wires ; connections to 1 10 volt D.C. circuit. 

Introductory : 

In the shunt dyjiamo the current generated in the 
armature divides, part of it passing through the coils of 
the field magnet, and the remainder passing out to the 
external circuit. In the most common type of ammeter, 
nearly all the current passes through a shunt, connected 
across the terminals of the galvanometer movement, and 
only a small fraction passes through the movement itself. 
In these and other cases of divided circuits, or shunts, two 
questions arise : How does the current divide between 
the two paths ? What is the combined resistance of the 
paths ? 

Experimental : 

Proper connections for a circuit of two branches, like 
that shown in Fig. 95, are to be made. The resistance in 

1 The lamps may be replaced by resistance boxes and the ammeters by 
tangent galvanometers, if only part (a) of the object of the experiment is 
to be worked out. 



DIVIDED CIRCUIT 



267 



each branch of the circuit consists of an equal number of 
similar incandescent lamps, connected in parallel. The 
ammeters are so connected that the total current through 
both branches can be read and also the individual current 
in each branch. The terminals of a voltmeter, which is 
not shown, are to be connected to the terminals of any 
portion of the circuit whose resistance is desired. 

All the lamps on both sides are to be turned on and 
reading of each ammeter recorded. The voltmeter is 



/v^v\ 



l w r 

Fig. 95. Lamp Board, Ammeters, and Connections. 



then connected in succession to the terminals of each 
branch circuit and to the terminals of the combined cir- 
cuit and the readings obtained recorded in tabular form 
near the top of the left-hand page. All the lamps but 
one on one branch are then turned out, leaving all the lamps 
in the other branch of the circuit burning. Readings of 
the voltmeter and ammeters are taken and recorded as be- 
fore. Make the following additional combinations in the 
two branches and record the results : 2 lamps and 3 lamps ; 
2 lamps and 4 lamps ; 2 lamps and 5 lamps. 



268 



LABORATORY EXERCISES 



Observations 



Branch A 


Branch B 


Total Circuit 


Lamps 


Amperes 


Yolts 


Lamps 


Amperes 


Yolts 


Amperes 


Volts 


5 
5 
2 








5 
1 
3 













2 
2 






4 
5 














A simple diagram of connections should be made, and a 
brief description of the method of making the tests should 
be given. 

From the readings of the instruments the resistance of 
each branch and the resistance of the entire circuit should 
be calculated for each case, by the application of Ohm's 
Law. The reciprocal of each resistance obtained should 
also be calculated to four decimal places. 



Calculated Results 



Branch A 










Lamps 5 
Resistance (i£ a ) 


5 


2 


2 


2 


1 











Branch B 










Lamps 5 
Resistance (i? 6 ) 


1 


3 


4 


5 


1 












RESISTANCE BY SUBSTITUTION 269 

Total Circuit 

Resistance (i?) - 

1 
B 

JL + J_ ..... ..... ..... 

Discussion : 

Does increasing the number of lamps in parallel in a 
circuit increase or decrease the resistance of the circuit ? 
When a number of equal known resistances are connected 
in parallel, give a rule for finding the combined resistance. 

Conclusion : 

(a) Complete the following statement : 

The currents in the branches of a divided circuit are 

to the resistances of the branches in which 

they flow. 

(5) Compare the sum of the reciprocals of the resist- 
ances of the branches of the circuit with the reciprocal of 
the resistance of the entire circuit. 



EXPERIMENT 78 

Resistance by Substitution 

OBJECT. To determine the resistance of a coil by direct com- 
parison with a known resistance. 

Apparatus. Galvanometer ; l resistance box ; reversing key 
(Fig. 97) ; Daniell cell, or dry cell ; two resistance coils, or other 
resistances, about 50 to 60 ohms ; copper wire for connections. 

x If a d'Arsonval galvanometer is used, it should be protected by a 
shunt or by a series resistance. 



270 LABORATORY EXERCISES 

Introductory : 

One of the simplest methods of measuring an unknown 
resistance is by direct comparison with a known resistance. 
When the same voltage is applied, the currents in two 
circuits will be the same if the resistances- are equal. The 
strength of two currents may be compared by the amounts 
that they deflect the needle of a galvanometer. 

After reading the deflection of the galvanometer when 
the unknown resistance is in circuit, various known resist- 
ances may be substituted for the unknown until the same 
deflection of the needle is obtained as with the unknown 
resistance. As the only difference in the two circuits lies 
in the resistance (unknown or knowm) inserted, equal de- 
flections mean that a known resistance has been inserted 
which is equal to the unknown resistance. 

Experimental : 

Arrange the apparatus as in Fig. 96. K\K% is a re- 
versing key (Fig. 97). The directions for the use of galva- 
nometers on pages 13 and 14 
should be read before using the 
instrument. 

(a) Close the key K x so that 
the current shall pass through 
the unknown resistance. Gently 
tap the galvanometer and read 
the deflection. Immediately 
open the key. 
Close the key K v so that, the current passes through 
the resistance box, from which one of the plugs has been 
removed. Why ? Remove plugs so as to obtain a total 
resistance which will give a deflection equal to that ob- 
tained with the unknown resistance, so far as the range of 
your box will permit. Keep the key depressed only when 






RESISTANCE BY SUBSTITUTION 271 

taking readings, and tap the galvanometer, as directed 
above. 

Again connect the galvanometer to the unknown resist- 
ance. If the reading is not the same as before, try to 
get a closer adjustment of the resistance box. Record 
the final readings of the gal- v K K* */ 

vanometer, connected through ^^^^^^u^^^^l 

the known and through the ^ „ 

. . Fig. 97. Reversing Key. 

unknown resistances. 

(5) Determine in a similar way the value of a second 

unknown resistance. 



Observations 

Deflection with unknown resistance 
Total known resistance in ohms . 
Deflections -with known resistance . 



Part A Part B 



Make a drawing showing the arrangement of the appa- 
ratus, and describe with reference to it the experimental 
method. State also the precautions to be observed with 
regard to the galvanometer and its readings. 

Discussion : 

Why is the circuit kept open, except when readings are 
being taken ? When the resistance box is in circuit, 
should the first resistance inserted be a large one or a 
small one ? Explain. State why a repeated comparison 
is made of the readings of the galvanometer through the 
known and through the unknown resistance. 

Conclusion : 

The resistance of ___' is ohms ; 

that of is ohms. 



272 



LABORATORY EXERCISES 



EXPERIMENT 79 



Heating Effect of an Electric Current 

OBJECT. To measure the number of calories of heat furnished 
by an incandescent lamp and to calculate the cost. 

Apparatus. Calorimeter ; thermometer ; 1 6 candle power 
incandescent lamp ; porcelain keyless socket ; voltmeter ; amme- 
ter ; source of 1 10-volt current ; graduate, or balance and weights ; 
flexible insulated wire for connections ; watch or clock with sec- 
ond hand. 

Introductory : 

Electrical heating devices are widely advertised and 
many of them extensively used on account of their con- 
venience. The common feature of them all is a well- 
insulated conductor of comparatively high resistance, 

made of a material capable of being 
heated to a high temperature with- 
out melting. The incandescent lamp 
has these properties and is sometimes 
used for heating purposes in " lumi- 
nous radiators. " By allowing a lamp 
to heat a known weight of water for 
a measured time, we may find the 
calories per second furnished by the 
lamp. If we know the current and 
voltage of the lamp, we may estimate 
the heat liberated per kilowatt hour. 
Although all the heat liberated by 
the lamp will not be measured in 
this experiment, yet the efficiency of the lamp as a heater, 
as used here, compares favorably 'with regular electrical 
heating apparatus. 




Fig. 98. 



HEATING EFFECT OF AN ELECTRIC CURRENT 273 




Experimental : 

A porcelain keyless socket is connected to a 110-volt 
line, with an ammeter between the socket and the 110-volt 
terminals (Fig. 98). A voltmeter is connected across the 
terminals of the socket. A lamp is then 
screwed into the socket and the switch 
closed in the circuit to make sure that the 
connections are correct and that the in- 
struments read in the proper direction. 
The lamp is then turned off till needed. 

Into a nickel-plated brass calorimeter 
is placed 250 grams (cm. 3 ) of water at a 
temperature six or seven degrees below 
room temperature. 1 This is stirred 
thoroughly with a thermometer and the 
temperature noted ; immediately the cur- 
rent is turned on through the lamp which 
is inserted in the calorimeter, the exact 
time in minutes and seconds being noted. The time and 
the temperature of the water are recorded in the tabular 
form near the top of the left-hand page, the voltmeter and 
ammeter also being read and their readings recorded. The 
lamp should be immersed until the tip rests on the bottom 
of the calorimeter, and the thermometer should stand in 
the calorimeter beside the lamp (Fig. 99). For the next 
five minutes the lamp burns inverted in the water. By 
moving the lamp up and down in the water, never raising 
it more than a quarter of an inch from the bottom, the 
water can be kept stirred and so of equal temperature 

1 This is the correct amount of water for the ordinary size calorimeter. 
The water should reach to within a quarter of an inch of the metal base 
of the bulb, when the latter is completely immersed. If the calorimeter 
is large enough to permit the use of a larger lamp, it should be used and 
the amount of water adjusted as just stated. 



Fig. 99. 



274 



LABORATORY EXERCISES 



throughout. The calorimeter should not be handled dur- 
ing the experiment. The voltmeter and ammeter should 
be frequently observed, and if there is any variation, the 
average reading for the whole time should be the one 
recorded and used. 

When the lamp has been in the water exactly five min- 
utes, take it out promptly, stir the water vigorously with 
the thermometer, and read and record the temperature. 

Using fresh quantities of water, repeat the test twice. 
The water equivalent of the calorimeter should be obtained 
from the instructor. 



Observations 





Trial 


Time 


Temperature 


Volts 


Amperes 


Begin 


End 


Begin 


End 


Begin 


End 


Begin 


End 


1 

2 
3 





























Weight of water 

Water equivalent of calorimeter 



9- 
9- 



Make a sectional drawing of the calorimeter with lamp 
and thermometer in place and with the connections of 
the instrument shown. A brief description of the method 
of the experiment should accompany the drawing. 

From the weight of the water, with the water equiva- 
lent of the calorimeter added, and the change of tempera- 
ture, the number of calories furnished in five minutes can 
be calculated. The number of watt-seconds is found by 
multiplying volts, amperes, and seconds together. From 
these two results calculate the calories per watt-second 



HEATING EFFECT OF AN ELECTRIC CURRENT 275 

and per kilowatt hour. As the time and the weight of 
water are the same in all three tests, the averages of tem- 
perature changes, volts, and amperes will be used in the 
calculation. The problem called for in the conclusion 
should be worked out in the note-book, using the local rate 
for electricity. 

Calculated Results 

Corrected weight of water {water + water 

equivalent of calorimeter') g. 

Average temperature change in five minutes . °C. 

Calories furnished in five minutes .... caL 

Calories furnished per second cal. 

Watt-seconds of energy used in five minutes . w.s. 

Calories per watt-second 

Calories per kilowatt hour 

Cost of current per kilowatt hour .... cts. 

Discussion : 

Explain any way in which heat generated by the lamp 
may escape without being measured in this experiment. 

Conclusion : 

At the price of cents per kilowatt hour, the cost of 

raising 4 liters of water from 15° C. to 100° C. will be 

cents, if an electric heater of the same efficiency as the 
lamp is employed. 



276 



LABORATORY EXERCISES 



EXPERIMENT 80 

Study of an Incandescent Lamp 

OBJECT. To measure the current, voltage, resistance, and power 
consumption of an incandescent lamp. 

Apparatus. Lamp socket, mounted on block with two bind- 
ing posts connected to the socket; 16 and 32 candle power in- 
candescent lamps ; low-range ammeter ; 120-volt voltmeter ; one 
or more lamps with the metal cap removed ; at least one tung- 
sten lamp, of known candle power ; $ 18 wire for connections to 
source of 1 10-volt current. 

Introductory : 

When we pay for electric light, we desire to get as 
much as possible for our money. We need to know the 
pressure required and the current consumed by our lamps. 
From these we can calculate the resistance of the lamp 
and the power in watts required to light it. By the use 

of a voltmeter and an ammeter 
properly connected to the lamp, 
we can observe the pressure and 
current directly. The resistance 
may be calculated by applying 
Ohm's Law. The watts are equal 
to the volts multiplied by the 
amperes. 



Filament 




Socket Contact 
Entrance 



Fig. 100. 



Experimental : 

Connect the ammeter in series 
with the lamp and the source of 
current. Connect the voltmeter to the terminals of the 
lamp socket, so that it will measure the fall of potential 
through the lamp only. Readings are to be made with 
16 and 32 candle power lamps, and the results worked 



STUDY OF AN INCANDESCENT LAMP 277 

out in each case. Readings with a tungsten lamp should 
be made by some members of the class. The results may 
be entered by the other members of the class for purposes 
of comparison. Assuming the candle power to be cor- 
rectly stated for the lamp, the number of watts required 
for each unit of candle power of the lamp should be cal- 
culated. This is known as the efficiency of the lamp, and, 
since power is what we pay for, it is used in comparing 
the economy of different kinds of lamps. 

The readings obtained should be recorded in tabular 
form near the top of the left-hand page. 

Observations 

CURRENT YOLTAGE 

16 candle power lamp .... amp. volts 

32 candle power lamp .... amp. volts 

candle power tungsten lamp . amp. volts 

A careful outline drawing, showing a vertical section 
of the lamp, with the parts labeled, should be made, in 
addition to the diagram showing the connections. 

At the top of the right-hand page place the results 
obtained by calculation. 

Calculated Results 

Resistance Power Efficiency 

7t 7 7 watts 

16 candle power lamp . ohms watts 

32 candle power lamp . ohms watts 

Conclusion : 



c.p. 
watts 



c. 



p. 



The average efficiency of a carbon incandescent lamp 

watts p t . watts 

is _ ; oi a tungsten lamp is — .• 

candle candle 



278 LABORATORY EXERCISES 

EXPERIMENT 81 

Lines of Force around a Conductor 

OBJECT. To investigate the magnetic field surrounding a con- 
ductor. 

Apparatus. No. 10 copper wire, bent at right angles and pro- 
vided with binding posts or double connectors at the ends ; dry 
cell or other source of current ; reversing switch ; jf 18 insulated 
copper wire for connections ; 4 small exploring compasses ; 2.5 
cm. compass; support which will permit the exploring compasses 
to be placed around the vertical portion of the wire, while the 
larger compass may be placed either above or beneath the hori- 
zontal portion. 

Introductory : 

When a current passes through a wire, magnetic effects 
may be observed in the vicinity of the wire. As such 
effects are always associated with the presence of lines of 
force, we wish to explore the field around the conductor 
to find the direction of these lines. This may easily be 
done by using compass needles, if we remember that a 
magnetic compass will set itself tangent to a line of force, 
and that a north pole will point in the direction of a line 
of force. 

Experimental : 

The direction of the current is from the + terminal of 
the cell, or dynamo, to the apparatus. Trace the current 
through the apparatus and back to the — terminal. 

1 Note to Instructor. The apparatus may be assembled permanently in 
the form shown in Fig. 101. The small compasses are set in holes bored 
in the block with a bit and cemented in place by rubbing them with a 
little shellac just before they are set in place. 



LINES OF FORCE AROUND A CONDUCTOR 279 



(a) We may determine the direction of the magnetic 
field around a conductor passing vertically through a block 
by placing small compasses on 
the block around the wire and 
observing their position, — 

(1) when there is no current 

flowing ; 

(2) when the current flows up; 

(3) when the current flows 

down. 




Fig. 101. 



The observations are to be 
recorded in three diagrams at 
the top of the left-hand page. 
In each, diagram the position 
taken by the small needles is to be shown by arrows 
in the four larger circles. The small circle in the cen- 
ter represents the wire. A current flowing up (toward 
the observer) is represented by a dot in a circle, thus O ; 
a current flowing down (away from the observer) by ®. 
These signs represent respectively the point of an arrow 
coming toward the observer and the feathers of an arrow 
going away from him. A sample diagram, showing the 
position of the needles in one case, 
is given in Fig. 102. 

(5) Place your apparatus so that 
the horizontal wire is parallel to one 
needle when no current is flowing. 
Place the compass under the wire 
and turn on the current. Observe 
the direction of deflection of the 
needle and record in diagrams, simi- 
lar to that shown in Fig. 103, placed in the upper part of 
the right-hand page. Note beside each diagram the posi- 





280 LABORATORY EXERCISES 

tion of the wire with respect to the needle (wire above or 
wire below}. The dotted arrow indicates the original 
position of the needle before the current passes and the 
solid arrow the position of the needle during the passage 
of the current. In all representations of the compass 
needle, the arrowhead indicates the north pole. 

Observe and record in the manner just 
described the four following cases : 

(1) Current S to N, wire over needle ; 

(2) Current N to S, wire over needle ; 

(3) Current S to N, wire under needle ; 

(4) Current N to S, wire under needle. 

Fig. 103. 

A simple outline drawing of the apparatus 

should be made on the left-hand page immediately below 
the diagrams of results, and a brief description of opera- 
tions written, referring to the drawings and diagrams. On 
the lower part of the right-hand page state the conclusions. 

Conclusion : 

(1) What is the shape of the lines of force around a 
straight conductor ? 

(2) Imagine the current as flowing in your right hand 
toward the fingers. If the palm faces the needle, toward 
what part of the hand is the needle deflected ? Make a 
full statement of this relation. 

(3) Suppose the wire to be grasped in the right hand, 
with the current flowing in the direction in which the 
thumb points. In what direction do the lines of force 
extend ? Make a full statement of this relation. 



THE ELECTROMAGNET 281 

EXPERIMENT 82 

The Electromagnet 

OBJECT. To study the construction of the electromagnet, and 
to determine the conditions of its operation. 

Apparatus. 1 Three electromagnet coils ; 2 a good dry cell ; 
single contact key; small box of half-inch brads; ft 18 wire for 
connections ; compass. 

Introductory : 

Doorbells, telegraph instruments, dynamos, motors, and 
many other kinds of electrical apparatus depend for their 
operation on electromagnets. These electromagnets con- 
sist of coils of wire, or solenoids, usually containing an 
iron core. We wish to locate the poles of such a magnet, 
to find the effect of the iron core on the strength of the 
magnet, and to find the effect of the number of turns of 
wire. Later experiments will take up applications of the 
electromagnet. 

Experimental : 

(a) Connect the terminals of the coil wound on the 
wooden core (Fig. 104, (7) to the dry cell through the con- 
tact key. By means of a compass needle, determine which 

1 The authors are indebted to Mr. W. R. Pyle, Morris High School, 
N. Y. City, for the plan of this experiment. 

2 Two of the coils are wound on}" soft iron cores and the third on 
\ !t dowel rod. The ends of the iron cores should be rounded off, as 
shown in Fig. 104, to increase the effect. On one of the iron cores (B) wind 
100 turns of # 22 insulated wire ; the ends of the coil are held in place by 
rubber rings, cut from a piece of tubing, with a slightly smaller internal 
diameter than the rod. After winding, the magnet is dipped in shellac, 
to hold the windings and rings in place. On the wooden core an exactly 
similar winding is placed and shellacked. The other iron core (A) is simi- 
larly wound, but with 50 turns only. 



282 LABORATORY EXERCISES 

end of the coil acts like a north pole and which end like a 
south pole. The key should be closed only when readings 
are being made, as otherwise the cell will rapidly polarize. 
Trace the direction of the current from the positive 
(carbon) pole of the cell through the coil, noting particu- 
larly the direction in which it flowed around the coil. 
Record this in the form of a simple diagram, showing only 

a very few turns of wire 
wound on the core, with 
an arrowhead on each 
to show the direction of 
the current, and with 
the poles marked. 

Grasp the coil in the 
right hand, with the 
fingers pointing around 
it in the direction of the 
current and the thumb 
extended. Does the 
thumb point in the direction of the north pole or in the 
direction of the south pole of the coil? State the relation 
in full in the Discussion. 

(5) Using the coil (Fig. 104, A) having the smaller 
number of turns wound on an iron core, test for polarity 
as in (a) . Does the presence of an iron core change the 
relation between the direction of the current around the mag- 
net and the location of the poles? 

Test the strength of the electromagnet by pushing one 
end into a box of brads, and then closing the circuit and 
removing the magnet with the brads which stick to it. 
Observe the behavior of the brads when the circuit is 
opened. What does this behavior indicate ? The brads 
picked up by the electromagnet should be counted and 
the number recorded. 




THE ELECTROMAGNET 283 

(<?) Determine the number of brads which can be picked 
up by the coil with the larger number of turns and the 
iron core (Fig. 104, i?), and record. Count the number of 
turns on each of the three coils. How is the strength of an 
electromagnet affected by the number of turns of wire it has ? 

(d) Try to pick up brads with the coil on the wooden 
core, and record the result. What effect has the use of an 
iron core on the strength of an electromagnet ? 

Record the numerical results obtained in tabular form 
near the top of the left-hand page. 

Observations 

Material of Core Number of Turns Number of Brads picked Up 



A brief description of the tests made should follow the 
table of observations and should include such observed 
results as are not stated in the table. A simple drawing 
should be made, showing one of the coils connected with 
the cell and key. 

Discussion : 

The questions in italics in the experimental directions 
should be answered under this heading. 

Conclusion : 

State the conditions necessary for a strong electro- 
magnet. 



284 LABORATORY EXERCISES 

EXPERIMENT 83 

The Electric Bell 

OBJECT. To study the construction and operation of the elec- 
tric bell. 

Apparatus. Electric bell with cover removed; dry cell ; push 
button; $ 18 wire for connections ; magnetic compass. It is de- 
sirable to bend the hammer rod so that the hammer does not 
actually strike the bell. 

Introductory : 

The electric bell is one of the most familiar applications 
of the electromagnet. A clear understanding of its con- 
struction, therefore, is of value to enable us to know what 
may be expected of the bell and what adjustments are 
necessary when it fails to operate properly. 

Experimental : 

(a) Connect the bell, the cell, and the push button in 
series, so that the bell will ring when the circuit is closed 
by the push button. 

(5) Trace the path of the current through the bell, 
starting at one of the binding posts. What draws the 
hammer toward the bell ? What draws the hammer away 
from the bell ? 

(<?) Place a compass needle near the ends of the mag- 
net coils. Hold the armature against the contact screw. 
Close the circuit and observe the result. Repeat with 
the armature held against the magnet poles. Is the mag- 
net stronger when the armature is pressed against the 
contact, or when it is against the poles ? 

(d) Detach the wire from the binding post on the 



THE ELECTRIC BELL 



285 



armature side of the bell, and press the end of the wire 
against the contact screw. Close the circuit. Note and 
explain the difference in operation. 

(#) On the right-hand page, make a full-size diagram 
of the instrument and its connections. Indicate by 
arrows the direction of 
the current at each im- 
portant point. 

Label the following 
parts : 

electromagnet cores 
contact screw 
spring 

electromagnet yoke 
vibrating armature 
push button. 

(/) Examine a push 
button and determine 
how the contact is made. 
Below the diagram of 
the bell, make a sketch 
of a vertical section of 
the push button and 
show the proper connections of battery, push button, and 
bell. 




Electromagnet 
■Yoke 



Porcelain 
Knob 



— Battery 




Spring 
Brass 



Push Button 

Fig. 105. 



Discussion : 

Explain the results of the tests in part (e). Why is the 
operation continuous when the circuit is closed ? 

What change in connections would convert this into a 
single-stroke bell, in which the gong is struck but once 
each time the circuit is closed ? 

Make diagrams showing the connections necessary for 



286 LABORATORY EXERCISES 

(1) two bells rung by one push button ; (2) two push 
buttons used to ring the same bell. Show the cell in 
each case. 



EXPERIMENT 84 

Telegraph Instruments 

OBJECT. To study the construction and operation of the instru- 
ments used on a telegraph line. 

Apparatus. Telegraph key and sounder ; dry cell or other 
source of current ; # 18 wire for connections ; at least one relay, 
properly connected in circuit with a key and sounder, for the labo- 
ratory, — if possible one for each laboratory table ; compass. 

Introductory : 

Joseph Henry, who made the first electromagnets in 
this country, suggested the possibility of sending signals 
to a distant point, and fifteen years later Morse con- 
structed the first practical telegraph line. The telegraph 
is, therefore, the earliest application of the electromagnet 
and one of the simplest and most useful electrical devices 
that we have. 

Experimental : 

(1) A telegraph key and a sounder, a cell, and connect- 
ing wires will be furnished you. These are to be con- 
nected in such a way that a current will pass through the 
sounder from the cell when the key is depressed. After 
satisfying yourself that the connection is properly made, 
the circuit should not be closed unnecessarily, as the noise 
is distracting to others. 

(2) Press the key ; observe and record the resulting 
movement in the sounder. If the instructor so directs, 



TELEGRAPH INSTRUMENTS 



287 



produce a dot, a dash, and any combination he may require, 
under his observation. 

(3) Trace the path of the current through the entire 
circuit. Holding the compass near the top of one of 
the electromagnet coils, 



Side Lever. 




l — 



=S\ 



£ 



Fig. 106. 



— ^ 

Insulated 

The Key. 






determine whether the 
magnet is stronger when 
the key is open or when 
it is closed. Account for 
the effect produced by clos- 
ing the key. Account for the effect ivhen the key is opened. 
Operate the short-circuit lever at the side of the key. 

(4) On the upper half of the right-hand page, make 
drawings of the key and of the sounder, seen from the 
side. Make a simple diagram showing the arrangement 
of battery, key, sounder, and connecting wires. Connec- 
tions in the instruments themselves, which are not exter- 
nally visible, may be indicated by dotted lines. Mark the 
following parts : In the sounder — magnet coils, soft iron 
armature, locker arm, pivot, spring. In the key — lever, 
pivot, contact points, spring. Indicate the position of any 
insulating material. Show by dotted lines the path of the 

current through base, pivot, 
and lever. 

(5) Examine the con- 
struction of a relay, if one 
is available. Trace the 
connections in its circuits 
and state what connec- 
tions are made from the 



Rocker Arm 



-Anvil 




Fig. 107. The Sounder. 



outside to each pair of binding posts. 

The drawings called for in (4) should be made first ; 
any additional description of the instruments should be 
placed on the left-hand page, accompanying a statement of 



288 LABORATORY EXERCISES 

facts observed in the examination an$ tests of the instru- 
ments. 

Discussion : 

Answer the italicized questions in the experimental 
directions as well as the following : 

Explain the use of the side lever in a line including two 
or more stations whose instruments are in series. How 
many keys can be in use in such a line at a time ? Why? 
Why can a relay be operated by a weaker current than a 
sounder needs ? Explain the use of a relay in a telegraph 
circuit. 



EXPERIMENT 85 

Operation of an Electric Motor 

OBJECT, (a) To observe the effect of a magnetic field on a cur- 
rent-bearing conductor ; (b) to study the construction and operation 
of an electric motor. 

Apparatus. Rectangular loop of # 28 spring brass wire, about 
10 inches long and 1-J- inches broad (Fig. 108) ; large U-shaped 
magnet, like those used in making magnetos or voltmeters ; 
2 storage or dry cells ; reversing switch ; 4 or 6 volt motor, with 
drum-wound armature, mounted so that the connection of the 
field leads to the armature can be reversed ; $ 1 8 insulated copper 
wire for connections. 

Introductory : 

The electric motor consists essentially of a coil of wire 
(armature) carrying a current, which rotates between the 
poles of an electromagnet. A commutator on the arma- 
ture shaft keeps the current flowing in a constant direc- 
tion through the armature. The wires on the opposite 



OPERATION OF AN ELECTRIC MOTOR 



289 



sides of the armature are caused to move by their lines of 
force seeking to become parallel with the lines of force of 
the field. By passing a current, first in one direction and 
then in the other, through a pliable wire located in a mag- 
netic field, we can imitate the action of the two wires 
forming the opposite sides of a coil on the armature. 

Experimental : 

(a) Pass a current through the loop of wire from a 
storage cell or dry cell. The circuit should be closed 
only when making tests. Bring the horseshoe magnet 
into such a position that the loop will be opposite the 
opening between the poles. Have the north pole of the 
magnet at the top, so that the magnetic field will be 
downward. Observe the behavior of the wire. 

Make a diagram, showing by a few lines of force in 
each case the field due to the magnet and that due to the 
current in the loop of wire. Indicate on 
the diagram, by arrows properly placed, the 
direction of the current, of the lines of 
force, and of the motion of the loop. Re- 
peat the test, with the direction of the cur- 
rent in the loop reversed. Record the result 
in another diagram. 

(6) Connect the field terminals and the 
armature terminals of the motor furnished, 
thus making it a shunt motor. Next con- 
nect the armature terminals through a re- 
versing switch to two or more cells in series. 
Close the switch, and observe the direction of rotation of 
the armature. Reverse the switch and again observe the 
direction of rotation. Keeping the direction of the cur- 
rent in the armature the same, change the direction of 
current through the field, by reversing the connection of 




Fig. 108. 



290 LABORATORY EXERCISES 

the field terminals. Observe the direction of rotation in 
this case. 

The results of the tests in part (a) are to be recorded 
in the two diagrams, which should be accompanied by a 
brief description of the operations. The results in part 
(5) should be stated in connection with the description of 
the work. This description should be accompanied by a 
drawing showing a side view of the motor, in which the 
following parts are shown and labeled: 
field magnet armature brushes commutator 

Discussion : 

Does this experiment illustrate the following rule for 
the motor ? " Let the forefinger of the left hand point in 
the direction of the magnetic field, and the second finger 
at right angles to the forefinger, in the direction of the 
current ; then the thumb will indicate the direction in 
which the current-bearing conductor will move." 

To reverse a motor, should both field and armature con- 
nections be reversed, or only one of them ? 



EXPERIMENT 86 

Power and Efficiency of a Motor 

OBJECT. To determine the horse power developed by an electric 
motor and the efficiency of the motor when developing that horse 
power. 

Apparatus. Electric motor, not smaller than \ H.P., with 
starting box and proper connections to a source of current ; canvas 
or leather strap, equal in width to the pulley of the motor, with 
two spring balances of 12 lb. capacity; suspension bar for the bal- 
ances and strap, with an upright support to which it can be clamped 
(Fig. 110); speed counter; watch; voltmeter and ammeter. 



POWER AND EFFICIENCY OF A MOTOR 291 

Introductory : 

In the selection of a motor, whether it is to run an auto- 
mobile or a sewing machine, the first consideration is to 
secure one that has the proper horse power. When a 
motor of the proper power has been found, then the effi- 
ciency with which it does its work should be investigated. 
Both the power and the efficiency of a motor can be 
measured with very simple apparatus. 

To calculate the horse power, it is necessary to find the 
number of foot pounds of work done per minute and di- 
vide this by 33,000, according to the definition of horse 
power. The number of foot pounds can be found by 
measuring with a spring balance the number of pounds 
friction between the motor pulley and a brake against 
which it turns, and multiplying this result by the total 
number of feet which a point on the revolving pulley will 
travel in one minute. The rated horse power of a motor 
or engine is the power it develops when working at full 
load; it does not develop this power at all times. 

The efficiency of a machine is the percentage of total 
work done on the machine which proves useful, or, since 
power is the rate of doing work, it is the percentage of 
total power applied to the machine which proves useful. 
By measuring the amperes of current flowing through the 
motor and the voltage applied to the machine, we can cal- 
culate the power applied in watts. Since 1 horse power 
is equal to 746 watts, the ratio of the power exerted by 
the motor on -the brake and the power applied to the motor 
by the current is readily found. 

Experimental : 

The motor, voltmeter, and ammeter should be connected 
to a source of current, as shown in Fig. 109, according to 
specific directions to be given by the instructor. Unless 



292 



LABORATORY EXERCISES 




a starting box is provided, the ammeter terminals should 
be short-circuited by a switch, which should not be opened 

until the motor has reached full 
speed. If a starting box is used, 
O \** J _l!i^ ^ e ammeter should be connected 

*a) between the source of current and 

the starting box, so that its read- 
ings shall show the current taken 
by both armature and field. The voltmeter, in any case, 
should be directly across the armature terminals of the 
motor. 

The brake consists of a strap, hung by two spring bal- 
ances from an adjustable support. By raising this sup- 
port until the bend in the strap is held against the under 
side of the motor pulley by the partly stretched springs of 
the balances, a frictional force is exerted on the surface of 
the pulley, the amount of which is equal to the difference 
between the readings of the two 
balances. 

The diameter of the pulley and 
the thickness of the belt in inches 
should be measured before the test 
is started and recorded in the tabu- 
lar form near the top of the left- 
hand page. A speed counter and 
watch should be at hand, ready for 
use, and the student who is to take 
the speed should be given specific 
directions by the instructor. When 
everything is ready, one student 
should take charge of the manage- 
ment of the brake and reading of the 
balances, a second should take the number of revolutions 
made in one minute, a third should watch the ammeter dur- 




Fig. 110. 



POWER AND EFFICIENCY OF A MOTOR 



293 



ing the minute and record its average reading, and a fourth 
should do the same for the voltmeter. 

(a) After the connections have been approved by the 
instructor, start the motor. Make sure that it is running 
in the right direction and that the voltmeter and ammeter 
are connected so that their needles read in the right di- 
rection. Adjust the tension of the brake by raising the 
supporting arm, so that the ammeter indicates about half 
as much current as the normal load of the motor requires. 
The voltmeter, ammeter, and spring balances should then 
be watched for one minute, while the speed is being taken, 
and all readings recorded. A second set of readings 
should be taken under the same conditions. If there is 
any marked variation in either voltmeter or ammeter read- 
ings during either minute, the results for that minute 
should be discarded and another reading taken. 

(J) Increase the tension of the brake, so that the motor 
takes the full number of amperes for which it is designed. 
Make two sets of readings, like those in (a), and record. 



Diameter of pulley 



Observations 

in. Thickness of strap 



m. 



Trial 


Balance Readings 


Speed 


Pressure 


Current 


High 


Low 


a-1 
a-2 
b-1 
b-2 


lb. 

lb. 

lb. 

lb. 


lb. 

lb. 

lb. 

lb. 


R. P. M. 

R. P. M. 

R. P.M. 

R. P. M. 


V. 

V. 

V. 

V. 


amp. 

amp. 

amp. 

amp. 



Make a diagram of the electrical connections and an 
outline drawing showing the brake in place on the pulley; 
write a brief description of the method. 



294 



LABORATORY EXERCISES 



Calculation of Horse Power. — The force exerted by the 
pulley on the brake is evidently the difference between the 
two balance readings, as when the pulley is turning there 
is more pull on one of the balances and less on the other 
than when the pulley is at rest, with the support of the 
brake in the same position. As a large portion of the 
pulley is always in contact with the brake, the distance 
through which the frictional force between the brake and 
the pulley acts in a minute is the same as the distance 
traveled by a point on the circumference of the pulley in 
a minute. In calculating this circumference, half the 
thickness of the belt is added to the radius of the pulley, 
and this measurement is reduced to feet. So the work 
per minute equals (difference between balance readings) 
X (diameter of pulley + thickness of belt) x tt x (revolu- 
tions per minute). Dividing the foot pounds per minute 
by 33,000 gives the horse power. 

Calculation of Efficiency. — The horse power obtained 
multiplied by 746 gives the power output in watts. The 
product of the volts and amperes gives the power input in 
watts. The former divided by the latter is the efficiency. 

The horse power and the efficiency at half load and at 
full load should be calculated, taking the average of 
the two readings in (a) for one calculation and the 
average of the readings in (6) for the other. 

Calculated Results 





Trial 


Net 
Force 


Distance 
per Min. 


Foot Pounds 
per Min. 


Horse 
Power 


Watts 
Output 


Watts 
Input 


Effi- 
ciency 


a 
b 


lb. 

lb. 


ft. 

_____ ft. 


ft. lb. 

ft. lb. 


H.P. 

H.P. 








/o 

/o 



POWER AND EFFICIENCY OF A MOTOR 295 

Discussion : 

Is the electrical energy consumed by a given motor 
independent of the work the motor is doing or dependent 
upon it? Does the motor always work at full horse 
power ? Is it better economy to select small motors for a 
factory and run them at full load, or large ones and 
run them at half load ? Would any energy be required 
to run a motor with no external load ? What would be 
the efficiency of a motor at no load ? Is the change in 
speed of your motor comparable in amount to the change 
in load ? 

Conclusion : 

The maximum horse power obtained from the motor 
tested was H.P. 

The efficiency at maximum horsepower was %. 

The efficiency of a motor is at full load than at 

partial load. 



296 



LABORATORY EXERCISES 



EXPERIMENT 87 



Relation between Fall of Potential and Resistance 

OBJECT. To determine the relation between the fall of potential 
in different parts of a circuit and the resistance of those parts of the 
circuit. 

Apparatus. High resistance wire, f 22 Prima Prima (la la), 1 
mounted on a meter stick ; voltmeter, low reading ; ammeter, low 
reading ; sliding contact ; dry or storage cells to give about 6 
volts ; wire for connections. 

Introductory: 

When a power house is delivering current to some dis- 
tant point, it is found that the voltage is higher at the 

power house than at the 
other end of the line. 
There has occurred a 
drop in voltage, which 
is equal to the pressure 
necessary to send the 
current through the line. 
By comparing the drop 
in voltage between the generator and different points 
with the resistance of the line between the generator and 
those points, the relation between the drop in any part of 
the circuit and the resistance of that part of the circuit 
may ba determined. 

1 This may be obtained from Hermann Boker and Company, 101 
Duane St., New York. German silver wire may be used instead, but its 
resistance is not so high as the la la, and the latter has a negligible tem- 
perature coefficient. 




Fig. 111. 



FALL OF POTENTIAL AND RESISTANCE 297 

Experimental : 

Connect a high resistance wire 1 meter long in series 
with an ammeter and storage cells. To the zero end of 
the wire connect the proper terminal of a low-reading 
voltmeter. Connect the other terminal of the voltmeter 
with a sliding contact. Examine all connections to see 
that the polarity is correct. 

Close the switch and place the sliding contact at the 
end of the wire opposite to the other voltmeter connection. 
Read the current, the length of the wire, and the potential 
difference between the ends. Move the sliding contact 
10 cm. toward the fixed contact and read as before. Re- 
peat, moving 10 cm. at a time until the fixed and movable 
contacts touch. 

Record all readings in tabular form near the top of the 
left-hand page. 

Observations 

1 2 3 4 5 6 7 8 9 10 11 

Length of resistance 100 90 80 70 60 50 40 30 20 10 

wire in cm. 
Potential difference 

between ends 

Current strength 

Make a diagram showing the connections, and, referring 
to the diagram, write a brief description of the method. 

From the laws of resistance, we may assume that the 
resistances of equal lengths of the wire are equal. By 
subtraction of each reading of length from the one preced- 
ing it, the change in length of the wire is found. By tak- 
ing the difference between each pair of successive voltmeter 
readings, the fall in potential for each of the 10 cm. 
changes in length is found. These subtractions should 



298 LABORATORY EXERCISES 

be made in order, beginning with the first two readings, 
and the results recorded in the table at the top of the 
right-hand page. In case the current changes during the 
experiment, calculate the resistance of each 10 cm. length, 
and record the changes in resistance instead of the changes 
in length. 

Calculated Results 



10 



Change in length of wire .. .. 

Fall of potential 

Discussion : 

What do the readings of the ammeter measure ? Those 
of the voltmeter? How could the actual resistance of any 
part of the wire be calculated? If the current remains 
constant, to what do you attribute the differences in the 
fall of potential in different lengths of the wire ? 

Conclusion : 

What is the relation between the fall of potential in 
different parts of a circuit and resistances of those parts 
of the circuit ? 



EXPERIMENT 88 

Resistance by the Wheatstone Bridge 

OBJECT. To measure the resistance of a conductor by means of 
the Wheatstone bridge. 

Apparatus. Wheatstone bridge, slide-wire form; galvanome- 
ter ; contact key ; plug resistance box ; Daniell cell, or dry cell ; 
wire for connections ; three coils or other pieces of apparatus for 
resistance measurement (resistance about 6 to 15 ohms). 






RESISTANCE BY THE WHEATSTONE BRIDGE 299 

Introductory : 

The Wheatstone bridge provides a rapid, accurate 
method of measuring a wide range of resistance (1,000,000 
or more ohms to a thousandth of an ohm with some forms 
of bridge). The theory of the bridge is based on Ohm's 
Law of the fall of potential in a circuit. The method 
of its use depends upon comparing the ratio between a 
known and an unknown resist- B 

ance, with the ratio between 
two known resistances. The 
four resistances are connected 
as shown in Fig. 112, AB con- 
taining the unknown resistance^ 
R v jB(7 a resistance box, R±, 
and AB and BO resistances, R 2 
and i? 3 , whose ratio is known. 

When B and B have the 
same potential and are con- 
nected through the galvanometer, no current will flow. 
Hence the needle is not deflected and the bridge is said to 

be balanced. The ratio ^ — — — ^ is the same as the 

it 4 (known) 





R\s$y 


N 


X 


A 


^<p 


A 




X\yv J Kl 
^\>\ \ /X 


yRs 


B71 


- ^5 


D 





Fig. 112. 



ratio. In the slide-wire bridge 



7? 

ratio —2, a known 
R s 

(Fig;. 113), A OB is a uniform wire, so — = — ^= — -r— -• 

v B J R R n length OB 

Thus, when the bridge is balanced, the lengths AB and 

OB are measured, the resistance R is known, and it is an 

easy matter to calculate the unknown resistance. 



Experimental : 

Arrange the apparatus as in Fig. 113, inserting a 
single contact key between one side of the cell and the 



300 



LABORATORY EXERCISES 



bridge. Notice that the current flows in a divided circuit 
through the bridge. Trace the current in each branch. 

Make the resistance in the resistance box 5 ohms for the 
first test. Close the key in the battery circuit, and then 
touch the wire at the 50 cm. point with the movable gal- 
vanometer contact. Observe the direction aud the amount 
of deflection of the galvanometer. It is not necessary to 
record these. Increase the resistance in the box by 1 ohm, 
and again test. Judge by the direction and amount of 

deflection whether the 

(G) R 



Ry 



Bh 



Fig. 113. 



resistance in the box is 
too much or too little. 
Change the resistance 
in the box until the de- 
flection is small, and 
then shift the movable 



contact until no current flows through the galvanometer. 
Record the resistance in the box and the distance of the 
sliding contact, as measured from each end of the metric 
scale, in a tabular form near the top of the left-hand page. 
Also specify the material and length or number of the coil 
whose resistance is being measured. 

Make a similar set of measurements for two other coils 
of unknown resistance. 



Observations 

Description op Kesistance 

Coil Measured in Box R Length AC Length OR 

ohms cm. cm. 

ohms cm. cm. 

ohms cm. ____, cm. 

Make a drawing showing the arrangement of the appa- 
ratus. Describe the method of balancing the bridge. 



RESISTANCE BY THE WHEATSTONE BRIDGE 301 

Let X represent the known resistance. When the re- 
sistance (J?) has been found for which no current flows 

X AC 

through the galvanometer, the proportion — = is true. 

R CB 

Hence X-—^— 

Calculate the value of each of the three unknown resist- 
ances. Record in tabular form at the top of the right- 
hand page. 

Calculated Results 

Description of Coil X Calculated Kesistance 

ohms 

ohms 

ohms 



Discussion : 

X AC 
Explain the relation — = — — by the principle devel- 

R CB 

oped in the fall of potential experiment (Experiment 87, 

p. 296). Under what conditions only will no current 

flow through the galvanometer circuit ? Show that this 

condition can be proved by Ohm's Law, since B = IR. 



302 LABORATORY EXERCISES 

EXPERIMENT 89 

Induced Currents 

OBJECT. To cause induced currents to flow through a coil of 
wire and to determine the laws of such currents. 

Apparatus. Coil of 100 or more turns of fine insulated wire, 
so wound that the direction of winding may be clearly seen ; sen- 
sitive galvanometer ; strong horseshoe magnet, such as is used in 
telephone magnetos, mounted in an upright position. 

Introductory : 

The dynamo, the induction coil, and the transformer 
illustrate the production of electric currents through 
closed circuits of wire which do not contain any voltaic 
cells. In each of these cases, an electromotive force is 
produced by moving coils of wire in such a way that they 
cut magnetic lines of force, or moving the lines of force so 
that they cut the coil. This process of producing an 
electromotive force is called electromagnetic induction. 

Experimental : 

Connect a coil of fine wire with a sensitive galvanom- 
eter. The terminal of the galvanometer at which the 
current enters to produce a deflection in a given direction 
must be known. Support a horseshoe magnet in an up- 
right position and move the coil rapidly downward over 
the north pole of the magnet. By means of a diagram like 
Fig. 114, record, near the top of the right-hand page, the 
direction of motion of the coil, the direction of winding of 
the coil, and the terminal of the galvanometer at which 
the current enters. This terminal should be marked -+- . 
After making this diagram, indicate by arrowheads the 



INDUCED CURRENTS 303 

direction which the current takes through the coil. The 
deflection of the galvanometer is to be recorded beside the 
diagram as "large" or "small." Observe whether the 
induced current continues after the motion of the coil has 
stopped. 

Allow the galvanometer to come to rest, then rapidly 
draw off the coil, observing and recording as before, by- 
means of another diagram. Repeat the test with 
the south pole. Record in a third and in a 
fourth diagram. Repeat any one of the tests, 
moving the coil more slowly. Record in a fifth 
diagram. Is the direction of deflection the same 
as when the coil was moved more rapidly ? Is 
the magnitude of deflection the same ? 

Place the coil over one pole of the magnet, 

and vary the magnetic field by suddenly pulling 

off the armature. This causes an increase in 

Fig. 114. 
the lines of force in the field surrounding the 

magnet. Record this result as before in a diagram. Next 

suddenly replace the armature, thus lessening the number 

of lines in the space around the magnet, and record this 

result. 

The seven diagrams take the place of a table of observa- 
tions. It is very important that each diagram be a com- 
plete record of the test recorded by it ; so be sure that the 
sign of the pole, the direction of motion of the coil, the 
direction of current in the coil, and the relative amount of 
current are indicated. 

No additional drawing is necessary, but the tests made 
should be briefly described. 

Discussion : 

How long i$ an induced electromotive force maintained ? 
What would be the probable effect of using a coil of a less 




304 LABORATORY EXERCISES 

number of turns ? When the coil is moving toward the 
pole of the magnet, does the pole of the coil attract or 
repel that of the magnet ? 

When the coil is moving off the magnetic pole, is there 
attraction or repulsion between its pole and that of the 
magnet ? 

Conclusion : 

How may an electromotive force be induced ? Upon 
what does the direction of an induced electromotive force 
depend ? Upon what does the amount of the induced 
electromotive force depend ? 



EXPERIMENT 90 

Study of a Dynamo 

OBJECT. To observe the generation of current in a simple 
dynamo and to study the construction of a direct current dynamo. 

Apparatus. Large U-shaped magnet, mounted with its poles 
vertical ; coil of fine wire wound on a wooden or an iron core, to 
be revolved between the magnet poles ; sensitive galvanometer ; 
two-pole, drum-wound small dynamo (6-8 volts); voltmeter; 
dry or storage cell, or other source of current for exciting the 
field of the dynamo. 

Introductory : 

Voltaic cells are not adapted to produce high pressures 
and large currents. Hence dynamos are employed in 
commercial work. Their action depends upon the fact 
that when a conductor cuts across magnetic lines of force, 
a difference of potential is produced in the conductor. 
The electromagnet which produced the lines of force in a 



STUDY OP A DYNAMO 



305 




dynamo is called the field magnet. The conductors which 
cut the lines are wires wound about a soft iron core. The 
wires and core together constitute the armature. The 
current generated in the armature is led out into the 
external circuit by means of brushes, which make a sliding 
contact with bars connected to the ends of the armature 
coils. These bars may be so placed and connected that 
the brushes change their contact 
from one bar to the next at just 
the instant when the difference of 
potential changes directi on. The 
bars then constitute the commu- 
tator. We shall first observe the 
generation of voltage in a simple 
dynamo and then examine a ma- 
chine of commercial type to locate 
the parts just described. 

Experimental : 

(a) Connect the long, flexible 
leads of the armature coil fur- 
nished you with the galvanom- 
eter (Fig. 115). Hold the arm- 
ature between the poles of the coil 
in a vertical position. Turn the 
coil sharply through a quarter of a revolution and ob- 
serve the behavior of the galvanometer. If you do not 
know from the direction of deflection of the galvanometer 
the direction of current flow in the two connecting wires, 
ask the instructor to show you how to determine this. 
In a diagram like that shown in Fig. 116, J., placed near 
the top of the left-hand page, record the direction of rota- 
tion, the direction of the magnetic field, and the direction 
of flow of current in the wires on each side of the armature. 



Fig. 115. 



306 



LABORATORY EXERCISES 



The other three tests to be made are to be recorded in 
similar diagrams, placed beside this one (Fig. 116; B, C, D). 
Turn the coil through the next quarter turn, observe 
the direction of deflection of the galvanometer, and record 
the results in a diagram like that for the first quarter turn. 
Repeat the process and make similar records for the third 
and fourth quarters of' the revolution. The current in- 
duced in this simple dynamo is an alternating current. 
In how many directions does the induced current flow during 
one complete revolution of the armature ? 




rw ****■ 




m 




Fig. 116, 




ltd] 



fffll 



(J) This part of the experiment should not be per- 
formed with the three-pole armature type of toy motor 
or generator. 

The following parts of the dj^namo should be located, 
and a sketch of the machine made showing them : field 
magnet, armature, brushes, commutator. Report on the 
following points of construction in your note-book : 

(1) Where and how the connection of one coil to 
another is made. 

(2) The connections made to each commutator segment. 
Connect the armature terminals to a voltmeter or to the 

galvanometer used in part (a). Supply current to the 
field from a dry or other cell. By twisting the armature 
with your thumb and finger, verify the statement that 
voltage is produced when lines of force are cut. Turn the 






STUDY OF A DYNAMO 307 

armature in the opposite direction and note the effect. 
Would the dynamo generate if the field magnet were con- 
nected to the brushes, instead of to some external source of 
current ? 

The description should include the results of any obser- 
vations not recorded in the diagrams called for above. 

Discussion : 

Answer the italicized questions occurring in the experi- 
mental directions. 

Show that the coils are so connected that the sum of 
the electromotive forces generated in them shall be the 
electromotive force at the brushes. 

Conclusion : 

State the use of each part of the dynamo, — magnet, 
armature, commutator, brushes. 



APPENDIX 



I. Important Numbers and Equivalents 

tt = 3.1416 
tt 2 = 9.8696 
Circumference of a circle = -n-D 



Area of a circle = ttt 1 or 



irD 2 



4 

1 centimeter = 0.3937 in. 
1 inch = 2.54 cm. 
1 mile = 1.609 kilometers 
1 cubic inch = 16.387 cm. 3 
1 pound avoir. = 453.6 g. 
1 ounce avoir. = 28.35 g. 
1 kilogram = 2.2 lb. 

1 liter = 1.0567 qt. (liquid) 
1 cm. 3 water at 4° C. = 1 g. 
1 ft. 3 water at 4° C. = 62.4 lb. 
1 atmosphere = 14.7 lb. 
1 atmosphere = 76 cm. mercury 
1 atmosphere = 30 in. mercury 
1 atmosphere = 33.57 ft. water 
Energy consumed in heating 1 lb. of water 1° F. (1 B. T. U.) 

= 778 ft. lb. 
Energy consumed in heating 1 g. of water 1° C. (1 calorie) 

= 3.09 ft. lb. 
1 British Thermal Unit (B. T. U.) = 252 calories 
1 horse power = 550 ft. lb. per second = 33,000 ft. lb. per minute 

= 746 watts = f kilowatt, nearly 
1 kilowatt = 1000 volt-amperes = I r ° T ° ir () - horse power = f horse 

power, nearly 
Heat in calories developed by resistance = 0.24 x amperes 2 x 
ohms x seconds = 0.24 watt-seconds 

309 



310 



APPENDIX 



CO 

l-H 

u 
<v 
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a* 

a 



w 
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u 

O 

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IH 

Oh 



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US 










iO xfi> 


52 


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£ 
















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Ph 


o 
















o 










o o 


H 


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1 


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S 


















d 










d d 


fa 






























o 

H 
































co 


00 


t^ 


Tt< 


i-h 


CM 


00 


00 


© 


rH 


© 




00 00 


fa 


(h 


CM 


rH 


t-H 


l-H 


r-i 


r-4 


cm 


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r-i- 


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q 


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PROPERTIES OF MATERIALS 



311 



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312 



APPENDIX 



III. Density of Water 



Grams per Cm. 3 



Temp. ° C. 


Density 


Temp. ° C. 


Density 


Temp. ° 0. 


Density 


0° 


0.999868 


11° 


0.999632 


21° 


0.998019 


1 


0.999927 


12 


0.999525 


22 


0.997797 


2 


0.999968 


13 


0.999404 


23 


0.997565 


3 


0.999992 


14 


0.999271 


24 


0.997323 


4 


1.000000 


15 


0.999126 


25 


0.997071 


5 


0.999992 


16 


0.998970 


26 


0.996810 


6 


0.999986 


17 


0.998801 


27 


0.996539 


7 


0.999929 


18 


0.998622 


28 


0.996259 


8 


0.999876 


19 


0.998432 


29 


0.995971 


9 


0.999808 


20 


0.998230 


30 


0.995673 


10 


0.999727 






100 


0.95838 



IV. Index of Refraction 



Water 1.33 

Alcohol 1.36 

Carbon bisulphide . . . . 1.64 



Crown glass 1.51 

Flint glass . . . . 1.54 to 1.71 
Diamond 2.47 



Electromotive Force of Cells 



Simple cell 1.0 volt 

Daniell cell 1.1 volts 

Gravity cell 1.1 volts 

Leclanche" cell . . . . 1.5 volts 



Dry cell 1.5 volts 

Bichromate cell . . . 2.0 volts 

Storage cell, lead . . . 2.0 volts 

Storage cell, Edison . . 1.2 volts 



NATURAL SINES AND TANGENTS 



313 



VI. Table of Natural Sines and Tangents 



Angle 


Sine 


Tangent 


Angle 


Sine 


Tangent 


Angle 


Sine 


Tangent 





0.000 


0-000 


31 


0.515 


0.601 


62 


0.883 


1.881 


1 


0.017 


0.017 


32 


0.530 


0.625 


63 


0.891 


1.963 


2 


0.035 


0.035 


33 


0.545 


0.649 


64 


0.899 


2.050 


3 


0.052 


0.052 


34 


0.559 


0.675 


65 


0.906 


2.145 


4 


0.070 


0.070 


35 


0.574 


0.700 


66 


0.914 


2.246 


5 


0.087 


0.087 


36 


0.588 


0.727 


67 


0.921 


2.356 


6 


0-105 


0.105 


37 


0.602 


0.754 


68 


0.927 


2.475 


7 


0.122 


0.123 


38 


0.616 


0.781 


69 


0.934 


2.605 


8 


0.139 


0-141 


39 


0.629 


0.810 


70 


0.940 


2.747 


9 


0.156 


0.158 


40 


0.643 


0.839 


71 


0.946 


2.904 


10 


0.174 


0.176 


41 


0.656 


869 


72 


0.951 


3.078 


11 


0.191 


0.194 


42 


0.669 


0.900 


73 


0.956 


3.271 


12 


0.208 


0.213 


43 


0.682 


0.933 


74 


0.961 


3.487 


13 


0.225 


0.231 


44 


0.695 


0.966 


75 


0-966 


3.732 


14 


0.242 


0.249 


45 


0.707 


1.000 


76 


0.970 


4.011 


15 


0.259 


0.268 


46 


0.719 


1.036 


77 


0.974 


4.331 


16 


0.276 


0.287 


47 


0.731 


1.072 


78 


0.978 


4.705 


17 


0.292 


0.306 


48 


0.743 


1.111 


79 


0.982 


5.145 


18 


0.309 


0.325 


49 


0.755 


1.150 


80 


0.985 


5.671 


19 


0.326 


0.344 


50 


0.766 


1.192 


81 


0.988 


6.314 


20 


0.342 


0.364 


51 


0.777 


1.235 


82 


0.990 


7.115 


21 


0.358 


0.384 


52 


0.788 


1.280 


83 


0.993 


8-144 


22 


0.375 


0.404 


53 


0.799 


1.327 


84 


0.995 


9.514 


23 


0.391 


0.424 


54 


0.809 


1.376 


85 


0.996 


11.43 


24 


0.407 


0.445 


55 


0.819 


1.428 


86 


0.998 


14.30 


25 


0.423 


0.466 


56 


0.829 


1.483 


87 


0.999 


19.08 


26 


0.438 


0.488 


57 


0.839 


1.540 


88 


0.999 


28.64 


27 


0.454 


0.510 


58 


0.848 


1.600 


89 


1.000 


57.29 


28 


0.469 


0.532 


59 


0.857 


1.664 


90 


1.000 


Infinity 


29 


0.485 


0.554 


60 


0.866 


1.732 








30 


0.500 


0.577 


61 


0.875 


1.804 


• 







314 



APPENDIX 



VII. Size and Resistance of Annealed Copper Wire 



B. AS. 
Gauge 


Diameter 
in Mils 


Area in 

Circular 

Mils 


Ohms per 
1000 Ft. 
at 20° C. 


Feet per 
Ohm at 

20° 0. 


Feet per Lb., 
Double Cot- 
ton Covered 


10 


101.89 


10,381 


0.997 


1,003 


30.9 


11 


90.74 


8,234 


1.257 


795.3 


38.9 


12 


80.81 


6,530 


1.586 


630.7 


48.8 


13 


71.96 


5,178 


1.999 


500.1 


61.5 


14 


64.08 


4,107 


2.521 


396.6 


77.4 


15 


57.07 


3,257 


3.179 


314.5 


97.2 


16 


50.82 


2,583 


4.009 


249.4 


121.9 


17 


45.26 


2,048 


5.055 


197.8 


153.1 


18 


40.30 


1,624 


6.374 


156.9 


191.5 


19 


35.89 


1,288 


8.038 


124.4 


246.9 


20 


31.96 


1,021 


10.14 


98.66 


297.9 


21 


28.46 


810.1 


12.78 


78.24 


374.5 


22 


25.35 


642.4 


16.12 


62.05 


471.7 


23 


22.57 


509.4 


20.32 


49.21 


584.8 


24 


20.10 


404.0 


25.63 


39.02 


729.8 


25 


17.90 


320.4 


32.31 


31.29 


901.0 


26 


15.94 


254.1 


40.75 


24.54 


1123 


27 


14.19 


201.5 


51.38 


19.46 


1389 


28 


12.41 


159.8 


64.79 


15.43 


1695 


29 


11.26 


126.7 


81.70 


12.24 


2127 


30 


10.02 


100.5 


103.0 


9.707 


2564 


36 


5.00 


25.0 


414.2 


2.414 


6666 



It will be noticed that the area of # 13 wire closely approxi- 
mates one half that of # 10, and that its resistance is twice as 
great. Throughout the table, an increase of three numbers cor- 
responds to doubling the resistance, and a decrease of three 
numbers to halving the resistance. 



WIRE CONSTANTS 



315 



VIII. Specific Resistance and Temperature Coefficient 

(From Timbie's " Elements of Electricity") 






Material (Commercial) 


Specific Resistance 

Ohms per Mil-Foot 

at 20° C. 


Temperature 

Coefficient = 

Increase per degree C. 

Resistance at 0° V. 


Aluminum 

Copper, annealed 

Copper, hard drawn .... 

Iron, annealed 

Iron, E. B. B. (Roebling) . . 

German Silver 

Manganin 

la la (Boker), soft .... 
la la (Boker), hard .... 
Advance (Driver-Harris) . . 


17.4 
10.4 

10.65 
90 
64 
114 to 275 
250 to 450 
283 
300 
294 


0.00435 
0.0042 

0.005 

0.0046 

0.00025 

0.00001 

0.000005 

0.00001 

0.00000 



SCIENCE 

First Principles of Physics 

By Professor HENRY S. Carhart, of the University of Michigan, and 
H. N. Chute, of the Ann Arbor High School. i2mo, cloth, 422 pages. 
Price, $1.25. 

THE present volume is more than a revision of the authors' 
popular High School Physics. It is a new book from cover 
to cover. No pains have been spared to make it mechanically 
the attractive volume which the increasing interest in the applica- 
tions of this practical subject deserves. The cuts number 457 
and will be found to constitute a prominent feature of the book. 
Especial attention has been given to the language, which has 
been made unusually simple and direct. The problems are nu- 
merous and interesting, and in them the difficulty of the actual 
arithmetical performance is reduced to a minimum, since it is 
recognized that the purpose of problems is the concrete illustra- 
tion of principles rather than practice in arithmetic. 

Although in keeping abreast of the times the authors have in- 
troduced many new features, they have been careful to retain the 
general scheme of presentation, and the just proportions, which 
made their former books so popular. The space given to the 
various topics is such as logical presentation demands. No topic 
is unduly emphasized in an effort at novelty of presentation. Each 
subject is treated concisely and is divided into numerous brief 
paragraphs with sub-headings, in order to aid the pupil in con- 
centrating his mind on the points of fundamental importance. 

It has been felt that many recent text-books in physics have 
sacrificed scientific and logical presentation in the effort to inter- 
est pupils by over-emphasis of some aspect of the science which 
has been considered attractive. The result of the use of such 
books has been a one-sided preparation and a consequent failure 
to meet college requirements. The authors of First Principles of 
Physics have shown that it is possible to produce a book which 
is as successful as their former texts in preparing pupils for col- 
lege and at the same time yields to no competing text-book of 
physics in attractiveness. 

62 



SCIENCE 



Laboratory Exercises in Physics 

By Raymond B. Brownlee and Robert W. Fuller, Stuyvesant 
High School, New York City. 

THIS Laboratory Manual is intended primarily to accompany 
Carhart and Chute's new First Principles of Physics, which 
it follows in the order of subjects. It is so arranged, however, 
that it can be used with any modern text-book in Physics. 

The Manual is the work of teachers in one of the best-equipped 
high schools in the United States and will be found up to date in 
every particular. 

There are eighty-nine experiments in the book. These cover 
a field so wide that from them may be selected a thorough course 
which can be given with the apparatus found in any school. At 
the same time the book affords enough material to satisfy teachers 
who have the best-equipped laboratories at their disposal. 

While the experiments meet the requirements of the College 
Entrance Board, particular effort has been made to adapt the 
work to the needs of pupils not preparing for college. 

The directions are simple and clear, and adapted to the ability 
of beginners in Physics. There are full instructions on the mak- 
ing of note-books. 

Elements of Chemical Physics 

By Josiah Parsons Cooke. 8vo, cloth, 751 pages. Price, $4.50. 

The Elements of Chemistry 

By the late Professor PAUL C. Freer, University of Michigan. i2mo, 
cloth, 294 pages. Price, $1.00. 

Chemical Tables 

By Stephen P. Sharples. i2mo, cloth, 199 pages. Price, #2.00. 



63 



SCIENCE 



First Principles of Chemistry 

By Raymond B. Brownlee, Stuyvesant High School ; Robert 
W. Fuller, Stuyvesant High School ; WILLIAM J. Hancock, Eras- 
mus Hall High School; MICHAEL D. SOHON, Morris High School; 
and JESSE E. Whits it, De Witt Clinton High School; all of New 
York City. i2mo, cloth, 425 pages. Price, $1.25. 

THIS book was prepared by the committee or teachers that 
was called upon to frame the syllabus in Chemistry for New 
York State. Its three fundamental features are : — 

1. The experimental evidence precedes the chemical theory. 

2. The historical order is followed as far as possible in de- 
veloping the theory. 

3. The practical aspects of the science are emphasized. 

In selecting their material the authors have been governed 
wholly by what they consider its intrinsic value to the elementary 
student, without reference to its traditional place in a text-book. 

To give the pupil some idea of the great commercial impor- 
tance of chemistry a number of typical manufacturing processes 
have been described and illustrated. When a substance is manu- 
factured in several ways the authors have given the process most 
extensively used in this country. The commercial production of 
copper, aluminum, iron, and carborundum has been described 
somewhat in detail, as these are notable examples of modern 
chemical processes. 

An important feature is the brief summary and the test exer- 
cises given at the end of each chapter. 

Laboratory Exercises to Accompany First Principles 

of Chemistry 

By the authors of the First Principles of Chemistry. 12m o, flexible 
cloth, 147 pages. Price, 50 cents. 

IN this manual are included seventy-one experiments, divided 
into three groups. Group A consists of forty-four experiments 
which all students should perform. Group B contains quantitative 
experiments, and Group C includes several extremely interesting 
experiments dealing with the practical applications of Chemistry. 

64 



SCIENCE 

Household Chemistry for Girls 

By J. Maud Blanchard, High School, Los Angeles, California. 
i2mo, cloth, 108 pages. Price, 50 cents. 

THE author's purpose is to outline a strong course in chemis- 
try, especially suited to girls of high school age. Though 
the ultimate aim is the training of intelligent homemakers, it is a 
manual of chemistry, not of domestic science. It is therefore 
suitable for a purely academic high school, no less than for a 
polytechnic high school, where a rigorous course in household 
chemistry forms a necessary foundation for the work in domestic 
science. The choice of subjects is based in a general way on the 
following scheme : — 

What we breathe. 

What we drink and use for cleaning. 

What we use for fuels and illuminants. 

Chemical nature of common substances. 

Foods and food values. 
• Adulterants and simple methods for their detection. 

Textiles — care of textiles, removal of stains, etc. 
The second half of the book, beginning with Experiment 28, is 
devoted to qualitative experiments in organic chemistry, as delicate 
quantitative experimentation is beyond the ability of high school 
pupils. Supplementary reading is of course advisable in this con- 
nection ; with this in view, a full list of library text-books is given, 
and definite references to these accompany the experiments. 

High School Physics 

By Professor Henry S. Carhart, of the University of Michigan, and 
H. N. CHUTE, of the Ann Arbor High School. New Edition, thor- 
oughly revised. i2mo, cloth, 444 pages. Price, $1.25. 

THE task of arousing interest, and of emphasizing especially 
attractive aspects of the science, is looked upon as the prov- 
ince of the individual teacher. This text-book aims simply at a 
clear-cut statement of general principles, giving each weight 
according to the scientific importance which it possesses. 

65 



SCIENCE 

Text-Book of Cooking for Secondary Schools 

By Carlotta C. Greer, East Technical High School, Cleveland. 
i2mo, cloth, ooo pages. Price, oo cents. 

THIS is not a book of recipes — it is literally a text-book of 
cooking, in which the practice of cooking is developed in a 
logical manner. The methods of cooking are practical, and the 
author shows the scientific principles on which they are based. 
Statements thus involving applied science are carefully kept 
within the understanding of pupils in the secondary school. 

The text-book is divided into two parts. Part I treats of The 
Cooking of Foods, Part II of Table Service and Food Values of 
Foods. Together the two parts furnish material for one year's 
work of four or five lessons a week, or for two years 1 work if the 
curriculum provides but two lessons a week. 

Part I is a guide to teach pupils to cook. The pupils follow 
established recipes and are taught to consider the processes of 
cooking as experiments in scientific study. Added to recipes 
and directions are suggestions to aid the pupil to appreciate the 
significance of each step and to understand the change that is 
taking place in the substances he is using. In the reviews the 
pupil is helped to work out his own scheme for preparing a meal. 

Part II adds to the planning and cooking of meals a practical 
method of calculating food values. Special attention is given to 
cooking and serving without a maid. 

The book is richly illustrated. 

The entire book has been worked out and tested in the class- 
room of one of the largest vocational schools in America. 

Descriptive Inorganic General Chemistry 

A text-book for colleges, by the late Professor PAUL C. FREER. Revised 
edition. 8vo, cloth, 559 pages. Price, $3.00. 

THIS is a text-book in General Chemistry for colleges and uni- 
versities. It aims to give a systematic course of chemistry 
by stating certain initial principles, and connecting logically all 
the resultant phenomena. 

m 



SCIENCE 

Practical Physiography 

By Dr. Harold W. Fairbanks, of Berkeley, California. 8vc, cloth 5 
570 pages, 403 Illustrations. Price, $1.60. 

THIS is the most attractive text-book on Physical Geography 
yet published. It contains over 400 illustrations, beautifully 
reproduced, and ten colored maps. Most of the views are from 
the author's own negatives, and were taken especially to illustrate 
parts of the book. 

The earth is not studied as a fixed model, but as a world 
whose physical features are undergoing continual change. These 
changes are seen to affect the climate and life conditions of plants 
and animals, and to have important influence on the activities of 
men. It is the object of the Physiography that the pupils gain 
an ability to understand the meaning of the phenomena of the 
land, the water, and the air, and the relation of all life to them. 

Part I treats of general physiographic processes. Part II has 
to do with the physiography of the United States. The book is 
intended as an aid to study -— not as a compendium of infor- 
mation ; consequently a description of the world as a whole is 
omitted. Attention is devoted specifically to the region of the 
United States, and typical examples afforded by it are studied as 
representatives of world-wide processes. 

No separate chapters have been devoted to the relation between 
physical nature and life, but instead, this relation is brought out 
in its appropriate place in connection with each topic throughout 
the book. 

The purely descriptive method has been discarded as far as 
practicable, the object being to lead the student to investigate 
and find out for himself. 

Suggestive questions are distributed throughout the book in 
close connection with the descriptive portions of the text to 
which they refer. The object of these is to stimulate the pupil 
to think for himself. 

Tne book includes field and laboratory exercises which may 
be enlarged and adapted to the needs of a particular locality. 

67 



SCIENCE 

Physics for College Students 

By Professor Henry S. Carhart, University of Michigan. 8vo, 
cloth, 631 pages. Price, $2.25. 

THIS is a new and widely successful text-book for a general 
course in Physics in colleges and universities. In writing it 
the author has kept constantly in mind those students who are 
not necessarily scientific in their taste or choice, but who desire 
a comprehensive outline of the leading features of Physics. 
Mathematical difficulties have been successfully reduced to such 
a degree that they may be readily surmounted by the average 
college student. 

The book contains a full treatment of Mechanics, Sound, Light, 
Heat, and Electricity and Magnetism. 



Physics for University Students 

By Professor Henry S. Carhart, University of Michigan. 

Part I: Mechanics, Sound, and Light. Revised edition of 1906. 
With 154 Illustrations. i2mo, cloth, 346 pages. Price, $1.50. 

Part II: Heat, Electricity, and Magnetism. Revised edition of 
1904. With 230 Illustrations. i2mo, cloth, 456 pages. Price, $1.50. 

THIS is a revised edition of the work which has for ten years 
been so favorably known to professors of Physics. The two 
volumes offer a more extended and more difficult course in general 
Physics than the Physics for College Students by the same author. 
Only such topics have been selected as appear most important 
to a general survey of the science. Somewhat more attention 
than is customary is given to Simple Harmonic Motion, because 
of its extensive application in Alternating Currents and its service 
in Mechanics, Sound, and Light. 

Although the treatment is often mathematical, mathematics is 
called into service not for its own sake, but wholly for the pur- 
pose of establishing the relations of physical quantities. 

68 



SCIENCE 

Exercises in Physical Measurement 

By Professors L. W. Austin, University of Wisconsin, and C. B. 
THWING, Syracuse University. i2mo, cloth, 208 pages. Price, $1.50. 

THIS book is a laboratory manual for the first year of the col- 
lege or university. 
Part I has those exercises which are in the practicum of the 
best German universities. They are exclusively quantitative, and 
the apparatus required is inexpensive. 

Part II contains suggestions regarding computations and im- 
portant physical manipulations. 
Part III contains in tabular form the necessary data. 

Elements of Physics 

By Professor Henry S. Carhart, of the University of Michigan, and 
H. N. CHUTE, of the Ann Arbor High School. i2mo, cloth, 408 pages. 
Price, #1.20, 

Electrical Measurements 

By Professor Henry S. Carhart and Professor G. W. Patterson, 

University of Michigan. i2mo, cloth, 344 pages. Price, $2.00. 

QUANTITATIVE experiments only have been introduced, 
and these have been selected with the object of illustrating 
general methods rather than applications to specific departments 
of technical work. 

Principles of Physics 

By Frank M. Gilley, of the Chelsea High School X2mo, cloth, 
560 pages. Price, $1.30. 

THE Principles of Physics is intended for use in the laboratory 
or classroom, or both. The author has made many im- 
provements on the apparatus hitherto in use, in many cases 
materially shortening the time in which the experiment may be 
performed, or facilitating its performance by large classes. 

69 



SCIENCE 



Anatomy, Physiology, and Hygiene 

By Jerome Walker, M.D. i2mo, cloth, 495 pages. Price, $1.20. 

THIS book has been written in the belief that the chief 
value of physiology in secondary schools is to teach care in 
the preservation of health. To that end the hygienic, rather than 
the biological, side of the subject has been given especial atten- 
tion, and there are chapters on Foods, Exercise, Ventilation, 
Clothing, and Bathing. It must not, however, be inferred that 
anatomy and physiology have been neglected. This part of the 
text has been revised in accordance with the latest investigations, 
and has had the criticism of eminent specialists. 

Herbarium and Plant Descriptions 

Designed by Professor E. T. NELSON. Portfolio, 7% x 10 inches. 
Price, 75 cents. Separate sheets, 80 cents per hundred. 

THIS is an herbarium and plant record combined, enabling the 
student to preserve the specimens together with a record of 
their characteristics. 

A sheet of four pages is devoted to each specimen. The first 
page contains a blank form, with ample space for a full descrip- 
tion of the plant, and for notes of the circumstances under which 
it was collected. The pressed specimen is to be mounted on the 
third page, and the entire sheet then serves as a species-cover. 
Each portfolio contains fifty sheets, which are separate, so as to 
permit of scientific rearrangement after mounting the specimens. 

The preliminary matter gives full directions for collecting, 
pressing, and mounting plants, as well as a synopsis of botanical 
terms. The cover is handsome and durable. 

Lessons in Elementary Botany for Secondary Schools 

By Professor THOMAS H. MACBRIDE, Iowa State University. i6mo, 
cloth, 343 pages. Price, 80 cents. 

THE student is not asked to study illustrations or text ; he is 
sent directly to the plants and shown how to study these 
and observe for himself the various problems of vegetable life. 

70 



MAR 1 1913 



